Ozone and the Reef Aquarium, Part 2:
Equipment and Safety

Ozone is often used by reef aquarists to "purify" the water. To most aquarists that means making the water clearer, and it certainly does that in many cases. How to optimally accomplish that task without risking the aquarium inhabitants' or the aquarist's health, however, is not always obvious. This article is the second in a series that discuss the details of ozone and its use in reef aquaria:

Ozone and the Reef Aquarium, Part 1: Chemistry and Biochemistry
Ozone and the Reef Aquarium, Part 2: Equipment and Safety
Ozone and the Reef Aquarium, Part 3: Changes in a Reef Aquarium upon Initiating Ozone

The series' first article detailed what ozone is and how it reacts with seawater. It also related ozone's perceived benefits to the actual chemical and biochemical changes that it can cause. In a sense, it provided the mechanistic framework for understanding why ozone does what it does and served to help aquarists understand its limitations.

This second article builds on these principles, using the mechanistic information about ozone's reactions to discuss how it is best employed in an engineering sense.

The sections are:


Figure 1 shows a schematic of how ozone is typically used in a reef aquarium. Some of these steps may be eliminated in particular applications, but aquarists should understand that by doing so they may be using other than optimal procedures. Subsequent sections of this article go through these steps one by one, detailing why each is important, how they are accomplished and the limitations to safe and effective ozone use.

Figure 1. A schematic of ozone's use in a typical reef aquarium system.

The process starts with an air source, usually a normal aquarium air pump. The air is often passed through a dryer where a hygroscopic material such as silica is employed that removes much of the water from the air; this is referred to as an air dryer. After passing out of the dryer tube and through an air check valve to prevent water from backing up into the system, the air enters the ozone generator itself. Drying the air in advance enhances the ozone generator's effectiveness.

After the ozone-laden air passes out of the ozone generator, it is sent to a mixing chamber where aquarium water and the gas are mixed well and are kept in contact for at least a few seconds. Aquarists often use skimmers or specially made ozone reactors for this purpose. Selection of suitable materials for these devices is a concern as the ozone can degrade some types of plastic, rubber and tubing.

Inside the contact chamber, the ozone reacts with many different chemicals in the seawater. Most of the benefits that accrue from ozone's use must take place in this chamber. Inside it, for example, the water is made "clearer" as certain light-absorbing pigments in dissolved and particulate organic molecules are destroyed, generally by oxidation.

Not all of the products of ozone's reaction with aquarium water are beneficial, however. Water leaving the contact chamber is optimally passed over activated carbon sufficient to remove the remaining ozone produced oxidants. The carbon breaks down most of these potentially hazardous oxidants before they enter the aquarium. The air passing out of the reactor also contains ozone and is also best passed over activated carbon to reduce the concern for airborne ozone's toxicity.

In order to ensure that not too much ozone or its byproducts enters the aquarium, aquarists monitor the aquarium water's ORP For those aquarists using a small amount of ozone, monitoring may be adequate. For those aquarists using large amounts of ozone, an ORP controller may be important. It can be used to shut off the ozone if the ORP rises above a set point (that point being either an emergency shut-off point that is rarely, if ever achieved, or a target ORP where the generator is actually running only part of the time and only when the ORP controller says that ORP needs to be raised to the set point).

Air Flow

Most ozone applications used by reef aquarists employ an air pump as their initial air source. While some units (such as one by Enaly) combine an air pump with an ozone generator, that is not the normal setup. Pressurized air in a cylinder or pumped tank, or even pure oxygen, can also be used, but due to their added expense those methods are unlikely to be used by most hobbyists. The only situation where aquarists might not use an air pump would be if the air/ozone mixture were being sucked through the ozone generator into a venturi, a common device on many skimmers, that allowed it to then enter a reaction chamber of some sort. In general, this is not the most common application, though, as an air dryer may put too much back pressure to allow a venturi to adequately draw in enough air.

How much air is enough? Luckily, it doesn't seem to matter too much. Sanders, a longstanding manufacturer of ozone equipment for aquarists, suggests on its website that air flow should be 50-500 liters per hour for ozone generators producing from 2 to 300 mg of ozone per hour. Larger units producing up to 2000 mg ozone per hour require airflow of 100 to 1000 liters per hour. Bear in mind that if the air is sent into a pressurized reaction chamber of some sort (as opposed to a skimmer), or even through a drying tube, substantial back pressure may reduce the air flow considerably below the rated maximum for an aquarium air pump.

Scientific studies have found that the air flow through corona discharge ozone generators does not seem to alter the production of ozone significantly unless the flow is slow enough that ozone produced inside the generator does not escape before it has a chance to be broken down by reactive species in the corona discharge (discussed below). One group1 fitted its results to the equation shown below:

X = Xo(1-e-a/F)

where X is the ozone concentration in the ozone generator's output in units such as mg/L, "a" is a constant relating to the unit's power, F is the flow rate and Xo is the maximum ozone concentration at low flow rates. The flow rate's effect on the ozone concentration is shown in Figure 2. It should be noted, however, that even if the ozone concentration is lower at higher flow rates, the total ozone produced is not. To find the rate of ozone production (in units such as mg/hour) requires multiplying the ozone concentration in the air produced by the rate of air flow (F):

Ozone Production Rate =  FXo(1-e-a/F)

The flow rate's effect on total ozone output is also shown in Figure 2. Note that it actually increases steadily with increasing flow rate. This effect is easy to understand. Higher flow sweeps away the newly produced ozone before it has a chance to break down again inside the generator and replaces it with fresh air containing O2, which is then ready to produce more ozone. Unfortunately, I do not know exactly where on these sorts of flow rate vs. ozone production curves that typical commercial aquarium ozone generators fall (or if they even follow this exact same relationship). Sander shows similar data on its web site for its ozone generators, with air flow rates of 0 to 600 liters per hour. The flow rate required to reach maximum total ozone production varies with the unit, but in all cases shown is more than 50 liters per hour, and for the larger units is more than 300 liters per hour. I do not know what flow rates all companies use to set the specifications of mg of O3/hr that are touted in sales literature, or if those flow rates used even match the recommendations that they provide to aquarists who use the devices. Such issues have been noted before in the literature2 where it can be difficult to compare commercial ozone generators without knowing the flow rates that were used when making the calculations.

Figure 2. The relationship between the air flow rate and the resulting ozone concentration (black) and the total ozone produced (red) for a typical corona discharge ozone generator.

Note that even if the commercial ozone generators used by aquarists produce a fixed amount of ozone per unit of time, the concentration in the air flowing through them will decrease as their flow rate increases.

In summary, the considerations with respect to air flow rate are:

1. Higher flow rates may mean higher total O3 production, maximizing the ozone generator's efficiency.

2. A higher flow rate means a lower concentration of O3 in the air. This reduction can lead to a lower transfer of ozone into the water (because the equilibrium amount entering the water depends on the concentration of O3 in the air). Large air volumes may also affect what sort of contact chamber is required to expose the tank's water to that air. Most can handle only a certain amount of air before malfunctioning, or at least decreasing the amount of water in it or the air's rate of turnover.

3. Higher flow rates may make it more difficult for ordinary drying tubes to adequately remove the moisture from the air before it gets to the ozone generator. Higher flow rates will also necessitate renewing the drying agent more often.

More comprehensive advice will be given at the end of the article, but my advice with respect to air flow is as follows:

1. Size an air pump so that it is in the range of flow rates recommended by the ozone generator's manufacturer, and perhaps also the contact chamber to be used. Perhaps use an air pump with a variable flow rate so that it can be adjusted during operation.

2. Use an air pump that can handle back pressure. How important this aspect is will depend on the nature of the pressure inside the contact chamber (next section).

3. Once the system is in operation, the air flow and other parameters can be adjusted to maximize performance. The aquarium's ORP is one easy, albeit slow, way to gauge performance. The ozone concentration in the water exiting the contact chamber, but ahead of the GAC, can be a good gauge. A chlorine or ozone test kit can be used to detect ozone and its byproducts in seawater since these compounds will react with the reagent in a standard chlorine kit. When using a Hach CN-70 chlorine kit (using the directions for either free or total chlorine), I found experimental values ranging from 0.02 to 0.5 ppm "chlorine equivalents" in different setups that I tried, not just varying air flow). Since such kits (which are based on a method called DPD or DDPD) detect a variety of different highly oxidizing species (hypobromite, ozone, etc.), it must be remembered that it is not an indication of just the total free ozone remaining. Nevertheless, the convention is to report all of these highly oxodizing species as if they were a single chemical (unless noted otherwise in a published study). The units can be chlorine equivalents or ozone equivalents, with 1 ppm chlorine equivalent equal to 0.7 ppm ozone equivalents (that value simply being the ratio of the molecular weight of O3 (48 g/mole) divided by the molecular weight of Cl2 (70.9 g/mole). Note that a test method using indigo blue (indigo trisulfonate) tests for ozone only, and not the byproducts, so do not choose that method unless you only want ozone measurements.

The ORP of the contact chamber effluent can also be a useful gauge (mine is typically in the upper 600's mV). In all cases, the higher the ozone or ORP, the more effectively the ozone is being used (at least when the flow rate of water through the reaction chamber is approximately constant).

Air Drying

Ozone generators using corona discharge operate most efficiently when the air entering them is dry. While the exact relationship between humidity and the ozone production rate depends on the generator's design, most commercial ozone generator manufacturers (O3ozone, Ozone Solutions and Lenntech, for example) show graphs of ozone production vs. humidity that look something like Figure 3. Many aquarists know the rule of thumb that ozone generation efficiency drops by about a factor of two between dried and undried air, and Sander makes a similar claim for its ozone generators on its website. Specifically, Sander claims that drying the ambient air with a relative humidity of 50% to dry air with a dewpoint of -40°C causes a 50% reduction in the ozone output of one of its line of ozone generators.

Data such as that in Figure 3 would seem to show that the maximum potential effect of drying is likely to be somewhat larger than two-fold if using ambient air, which can have dewpoints running up to 20°C or even higher, compared to very dry air (with a dewpoint below -60°C). For convenience in interpreting Figure 3, the table below shows the relationship between relative humidity and dewpoint when the air temperature is 70°F (21.1°C). Obviously, the air must be very dry to have a dewpoint below -20°C. It is not obvious, however, whether the sorts of air dryers used by hobbyists approach or exceed this low dewpoint.

Table 1. The relationship between the dewpoint and the relative humidity at 70°F (21.1°C).
Relative Humidity
Dewpoint (°C)
Figure 3. The relationship between the dewpoint (humidity) and the relative amount of ozone produced in a typical corona discharge ozone generator.

It is also claimed that higher humidity in the incoming air can increase the output of nitric acid, but not all researchers agree on this assertion.2 Some resources3 recommend that the dewpoint be kept very low (~-60°C) in order to prevent corrosion of the unit itself by nitric acid's formation inside it. Again, however, it is not obvious whether the sorts of dryers used by hobbyists approach this very low dewpoint.

One aquarist reported corals in his aquarium started looking poorly, and discovered that there was a blue liquid in the tubing between his ozone generator and a brass fitting. he had not been using an air dryer, and it was a humid day. That liquid may well have been nitric acid in water that corroded the brass fitting to release copper, that then made its way to the aquarium. A more extensive discussion of the chemistry behind nitric acid formation is presented in the next section.

In any case, most ozone generator manufacturers suggest that the air be dried before it enters the generator, and aquarists have several options for drying the air. Some commercial devices can dry air rapidly and automatically, although they are considerably more expensive than other options. These commercial devices are especially useful in high air flow applications (many liters per minute).

The simplest dryer is a plastic tube filled with a material that binds to moisture in the air. The air flows in one end and out the other, and gets dried while passing through. Red Sea sells such a device in at least two sizes. Their material (silica gel) changes color from blue to pink as it is exhausted, and it can be regenerated in a standard oven by warming it up, thereby driving off the absorbed water. Unfortunately, my device came missing a critical O-ring, and when I resorted to making it myself, the unit sometimes could not hold adequate pressure. It also seemed to become depleted faster than I had hoped. In my system I used the larger size (500 g), but found that it typically became depleted in two weeks or so. That result is apparently mirrored by others' experiences, so anticipate such a discharge period. Nevertheless, the color changing ability makes depletion apparent. I also found a surprisingly small effect of using the dryer on ozone in the effluent from the reaction chamber, and on overall aquarium ORP. Details of that finding will be discussed next month, but that result may reflect a lack of effectiveness of the drying tube, or alternatively, a lack of a large effect of humidity on the ozone production by the Aquamedic ozone generator that I used.

Some aquarists use two units in series, so one can be swapped out for regeneration while the other is still in place. Figure 4 shows the setup used by Jose Dieck, in which he has drying tubes mounted on a wall with quick disconnects to permit rapid swapping in and out as necessary. In addition to simplifying the replacement process, such a setup may drive the dewpoint lower than a single pass system using the same tubes.

Figure 4. The ozone generation setup used by Jose Dieck, showing two drying tubes used in series.

Do-it-yourselfers may be able to buy silica gel themselves and fashion a drying tube. Other materials might work, but may entail complications. Damp-Rid, for example, may actually liquefy in the presence of too much moisture, and it may also not reduce the humidity enough.

No Dryer

Ozone generators using UV light to generate ozone (e.g., Ultralife) require no drying of the source air. In addition, many aquarists using corona discharge ozone generators just skip the air dryer when using ozone, and seem to be happy with the ozone's usefulness in their setup. The fact that they may be getting only 50%, 10% or even 2% of the rated output may not be important to them. If the aquarium's ORP rises enough without a dryer that an ORP controller is actually "controlling" it by shutting off the ozone for some portion of the time, then the ozone production is obviously adequate. Likewise, if the ORP is such that the aquarist has dialed back the O3 generation setting on the generator to less than maximum, and is happy with the results, then a dryer would not likely be especially beneficial.

Water clarity may improve at levels of added ozone far less than required to raise the ORP to the often mentioned 350-450 mV range. In the end, all that matters is that the aquarist is satisfied with the water's clarity and with whatever other expectations he has for its benefits. The undesirable effects of nitric acid production (slight additions of nitrate, slight reductions in alkalinity and pH) are likely trivial compared to the huge additions of nutrients and buffers that many reef aquaria experience.

Will the inside or the fittings of a corona discharge ozone generator unit degrade over time due to nitric acid corrosion? I do not know the answer to that.

In my setup the ORP never rises above 330 mV, and is more typically 300-330 mV even with the ozone unit that I have turned to its highest setting, and with an air dryer (all of which I will detail next month). This result suggests to me that I am nowhere near overdriving the ozone addition. For this reason it would seem prudent to continue to use a dryer, but the actual experimental results that I obtain over the coming months (where humidity is likely to rise further) will determine if continued use of the drying tube is warranted going forward.

My advice to others with regards to air drying is:

1. More ozone may be produced by the ozone generator if the air is adequately dried first, assuming it is a corona discharge type. It remains to be established, however, whether simple commercial air drying units have the desired effect.

2. The ozone generator itself may last longer if the air is adequately dried (again, assuming it is a corona discharge type).

3. Assuming that water clarity is the primary or only goal of using ozone, and not the more difficult to achieve goal such as disinfection of the water, many aquarists will likely be satisfied using ozone without an air dryer.

Air Flow Check Valve

An air flow check valve is an inexpensive and potentially important piece of equipment. It can be used between the dryer and the ozone generator, or between the ozone generator and the ozone reaction chamber. Being a high voltage electrical device, ozone generators do not mix well with seawater. While many seem able to withstand occasional water contact (and some even recommend cleaning inside the air passage with distilled or RO/DI water), deposits of salts and other materials is likely not desirable. Even if the ozone generator is located higher than all other pieces of equipment, some ozone reaction chambers have enough pressure in them that if the air flow stops, water can back up in the air line to a considerable extent.

If used between the dryer (or air pump) and the ozone generator, any check valve is adequate. If air cannot move backward through it, then in a power failure when the air pump turns off, water cannot come up the air line tubing into the generator. In this setup, water can come up the tubing if the air line between the generator and the check valve somehow comes off.

If used between the ozone generator and the ozone/water reaction chamber, an ozone resistant check valve is preferable (if one can be found). In this setup, water cannot reach the ozone generator as long as the check valve is in place. In the absence of ozone resistant valves regular check valves can be used and swapped out frequently as the rubber in them degrades due to ozone exposure. The materials that are most suited to surviving ozone exposure are detailed later in this article.

Ozone Generators: Electric Discharge Theory

Ozone has historically been generated in a variety of ways for aquarium applications. These include high energy UV radiation and electrical discharges. Most, but not all, commercial ozone generators intended for aquarium use employ electrical discharge. Figure 5 shows a typical electrical discharge unit. In it, air is passed between two electrodes. An alternative design is to simply have the air pass through a glass tube that is between two electrodes. While any charge separation across the electrodes can work, an AC (alternating current) field is often used. The exact nature of the electrical field varies, and usually falls into one of the following frequency ranges: low frequency (50 to 100 Hz), medium frequency (100 to 1,000 Hz) or high frequency (1,000+ Hz). I am not sure what frequencies are used in each of the commercial brands commonly employed by aquarists. A thin dielectric material is coated on one or both electrodes to prevent actual sparking between the electrodes. That dielectric material can be glass, mica or other nonconductive materials, but is usually glass. The electric field between the electrodes is strong enough to rip apart molecules and is called a corona or corona discharge. Coronas often emit light, and while that effect cannot be seen in typical commercial ozone generators, it can be seen in other applications where the corona is not so enclosed.

Figure 5. A schematic of the internal workings of a corona discharge ozone generator.

The intense electric field, and the high energy ions within it, can rip apart all of air's primary components into very reactive individual atoms or radicals:

N2  à  2N

O2  à  2O

H2à  H + OH·

These species can then react among themselves, or with unreacted components in the air. It is beyond the scope of this article to detail plasmal chemistry, but the reaction of most interest to us is:

O +  O2 à O3 (ozone)

As mentioned above, the air flow through the generator can impact the amount of ozone produced. With an understanding of how ozone is produced in such generators, it is easy to see why. If O3 is produced between the electrodes, and sits there for a period of time, the ozone itself can be ripped apart by the intense electric field and by collisions with high energy electrons and other species:

O3 à O2  +  O

A higher air flow rate can help to sweep the initially formed ozone out of the generator before it can be broken apart, and to replace it with fresh O2 that is ready to produce more ozone.

Several reaction sequences can result in nitric acid:

O2 + H à  HO2· 

N +  O à  NO· 

HO2·  +  NO· à  HNO3 (nitric acid)


N  +  O2 à  NO2· 

OH· + NO2·  à  HNO3 (nitric acid)

The last sequence requires that water be present (to get to OH·) and it's apparent how a high water concentration (as indicated by the humidity or dewpoint) might increase the nitric acid concentration.

These sorts of processes can also explain how high water concentration in the air (i.e. high humidity or dewpoint temperature) might decrease ozone production. Instead of reacting with O2 to produce ozone, for example, an oxygen atom can react with the breakdown products formed from water (H and OH·) to produce other chemicals. Other reactions among these species also lead to products such as hydrogen peroxide and nitrous acid, but they are lower in concentration than oxygen.

Ozone Generators: UV Theory

As mentioned above, ozone can also be generated by intense ultraviolet light. The ozone generators sold by Ultralife fall into that category. These devices use a special light bulb producing short wavelength UV light (often 185 nm). During UV exposure at this wavelength, O2 molecules in air passing near the bulb absorb the light and are broken apart:

O2  à  2O

As with electric discharge units, these oxygen atoms can then combine with O2 to form ozone:

O +  O2 à  O3 (ozone)

The manufacturers of these types of units claim that their advantages are that the air need not be dried, and that fewer nitrogen-containing byproducts are formed (e.g., nitric acid). Additionally, their bulb is said to last for two to three years before needing replacement. Competitors have claimed that these types of ozone generators lose about 20% of their rated output after a few hours of operation, and that the electrical power consumption is much higher for a UV based system than for corona discharge. The maximum concentration of ozone that can be obtained in a given air volume is lower (01 - .1% by weight O3 in air for UV systems compared to 0.5 to 1.7% O3 in air for dried air using corona discharge). Note that the UV type ozone generators' output often is not adjustable.

Also noteworthy is that these units are distinctly different from UV sterlizers. Ultraviolet sterilizers use a longer wavelength of UV light (about 254 nm, typically) and kill organisms by UV's direct interaction with the tank's water as it passes by. Molecules such as DNA in the organisms absorb the 254 nm UV and the molecules break apart, killing them. Ultraviolet light at 254 nm does not produce significant ozone.

Which type of ozone generator is better? I chose a corona discharge type for my setup, but either method is adequate for most hobbyists.

Ozone Generators: Practical Information

As a practical matter, ozone generators are easy for aquarists to use. If their ozone output is adjustable, the device will have a control dial on it. Such a dial controls the power applied across its internal electrodes. Otherwise, there is nothing to set or adjust (unless the ozone generator comes packaged in a box with a redox controller, which is discussed below). If they are not adjustable, they may have nothing more than an electric cord, an air inlet and an air/ozone outlet.

Ozone generators for aquaria that use a corona discharge consume very little electricity. Typical aquarium units use 10 watts or less (for 300 mg O3 per hour or less). They usually come with adequate directions for their use. Ozone generators frequently used by aquarists in the United States include those made by Sander, Aqua Medic, Enaly and Red Sea. Units based on UV light (e.g., Ultralife) typically use more electricity.

Gauging how much ozone is necessary is not trivial, and may depend strongly on the desired outcome from dosing ozone, how it is used and the other husbandry practices used in the aquarium. Clearing up yellowing in the water, for example, uses far less ozone than is necessary to sterilize the water. Likewise, a good ozone/water reaction chamber might allow far less ozone to be used than is required by an inefficient use in a skimmer. That being said, most guidelines suggest on the order of 0.3 to 0.5 mg O3/hr/gallon of aquarium water.

If possible, I would suggest locating the unit above the water's level where it is being used. All sorts of malfunctions (power failures, air pump failures, loose air line, etc.) can send water back up the air line tubing and into the ozone generator. Such water contact may not immediately ruin a corona discharge unit, but it will contribute to poor output and may eventually cause it to quit functioning. I am not sure what effect contact by liquid water would have on a UV based ozone generator, but it would not surprise me if it could shatter the bulb. An air check valve also helps reduce the likelihood of water contact. I have my Aquamedic ozone generator attached about 7' off the floor of my basement, where the treated water is sent into the reaction chamber and ultimately into the sump that is about 3-6' lower. Nevertheless, I have accidentally sent water into my ozone generator several times. In each case, the amount of ozone in the reaction chamber seems to come back to normal after 24 hours, but this practice is likely less than desirable.

Check with the manufacturer or the supplied directions before attempting to clean the inside of an ozone generator. Some recommend cleaning with pure fresh water and a brush, but that is not possible with other designs. My Aquamedic unit is sealed with a membrane of some sort, so poking any solid object into the fittings will damage it.

Ozone Reaction Chamber: Skimmers

The ozone reaction chamber is the heart of the system. It is the place where air, laden with ozone, and water from the aquarium are mixed together. In the first article in this series I detailed the chemistry and biochemistry that occur in the reaction chamber. I also discussed issues relating to contact time and ozone concentrations with respect to some of ozone's potential effects (such as disinfection).

A variety of different systems can be used as contact chambers, and most reef aquarists choose to use skimmers. They use either their main skimmer or a smaller, inexpensive one that can run at a lower flow rate and potentially be sacrificed if the ozone degrades the plastic to the point where it no longer is reliable. Despite their widespread use with ozone, skimmers are not usually an optimal way to employ ozone for several reasons:

1. Their water and air flow rates, and even their engineering design itself, are optimized for skimming, not for ozone injection and reaction. The longer the ozonated water has to react, the more oxidation of organic molecules can take place. This is not a design criterion with skimmers, where the air/water contact time is maximized, but the water alone is not held for any purpose. If the water's flow rate is too high, and hence its turnover rate too high, the concentration of ozone in the water, and the contact time for it to react with organic materials, may be less than optimal.

2. Both the air and water exiting the skimmer should optimally be passed over activated carbon to reduce the highly oxidizing and toxic species being sent into the aquarium and into the aquarists' home air. Many skimmers are not set up to efficiently pass the air over carbon, and high water flow rates can make it difficult to achieve adequate contact with activated carbon.

3. Many skimmers are not designed using materials suitable for prolonged ozone exposure.

Nevertheless, the majority of reef aquarists who use ozone do so with a skimmer. Whether it is optimal or not, they have decided it meets their needs. How ozone is used with a skimmer depends critically on the nature of the skimmer, and too many different designs exist to provide many useful details. However, some suggestions for using ozone this way are:

1. Select a skimmer that allows a substantial volume of water to be contained within it, so that the ozonated water is not immediately swept away and passed over the GAC (where the ozonation reactions largely end).

2. Select one that lets you collect the air and pass it over GAC. A Sea Clone, for example, would be a poor choice in this regard as the air and water exit it from a fairly large opening. The ETS skimmer that I use is also a poor choice, as the air comes out of a tube that is also the skimmate outlet. It can, however, be used with a special skimmate collector (described below).

Jose Dieck has modified a commercial skimmate collector (PS-MQWC2) that works in conjunction with his skimmer. He made a new cap, extended the length of the neck between the top flange and the carbon container and re-tapped the flange to accept a larger ¾" fitting for the drain. Originally, the carbon was intended to remove the skimmate's smell, but it can also work to reduce ozone. It allows the liquid skimmate to be collected and diverts the ozone-laden air through an activated carbon filter (Figure 6). It requires the skimmate to be drained by gravity from the skimmer cup to the collector chamber without releasing any of the air. The air/skimmate mixture enters at the top, the liquid settles to the bottom and the ozone laden air comes out through the middle of the top. It passes over carbon, thereby losing its ozone. It can also be vented outside, as desired.

Figure 6. A modified skimmate collection container that is used by Jose Dieck to reduce airborne ozone release.

Ozone Reaction Chamber: Commercial Reactors and DIY

Several commercial ozone reactors are available, which range from poor to what is likely quite effective (albeit expensive). I have used the Coralife Ozone Reactor (Figure 7), and won't use it again. In my opinion it is not a well-designed product. I'll provide more commentary on it next month.

Figure 7. The Coralife ozone reactor with attached tubing for water and air flow.

Marine Technical Concepts (MTC) also makes an ozone reactor, the PRO240D. It consists of a 6" diameter acrylic tube that is 27" tall. Inside the water is dripped through a plate and then onto a high surface area plastic material. The air/ozone mixture is injected above the plate allowing them to mix. This type of reactor is typically pressurized to several PSI, driving the ozone into the water. I've not used it, but I am confident that this reactor would be a good choice.

Those who want an ozone reactor but who are not able to spend several hundred dollars might use the PRO240D or these linked plans as guides for DIY (do-it-yourself) systems.

Ozone Reaction Chamber: Tubing Reactor

After messing with the Coralife Ozone reactor and finding it unsatisfactory, and doing some tests where I simply sent the ozone into my skimmer (making my basement stink of ozone), I decided to set up a very simple "reactor" myself (Figure 8). I have two Iwaki 30 RLXT pumps in series that I have used for years as my main return pumps. I created a "T" off of their output to send water to my two main tanks.

Figure 8. The 100' coil of HDPE tubing that I used as a simple ozone reactor.

Using another "T" I added a ¾" venturi, and to it I attached a 100' coil of ¾" HDPE (high density polyethylene) tubing that I bought from Cole Parmer for about $60 (including shipping). The reactor simply consists of the air/ozone mixture pumped into the venturi, and then the water/air/ozone mixture circulates through this coil (about 13 individual coils) for about 45 seconds (when the water's flow rate is about 90 gallons per hour). It contains a little over two gallons of air and water at a time. This allows for a long contact time with a significant amount of water, and a fair amount of pressure exists both from gravity and from the back pressure of 100' of coiled tubing. In fact, the tubing coil had to be laid horizontally. Hanging it vertically created too much back pressure to get any significant water flow through it.

While the mixing efficiency is apparently not especially good inside the tubing, it is adequate to raise the ORP to > 680 mV and the ozone concentration in the water (as measured with a chlorine kit at the outflow) to 0.1 ppm chlorine equivalent. In this setup, the venturi simply acts as an inlet for the pumped air because the flow rate is too low to actually get any suction by venturi action.

Most important to me, the end of the tubing where the air and water exit is easily passed through a column of GAC to remove residual ozone and ozone by-products. In normal operation I smell no ozone in the basement room where the operation takes place. There is also no place for any detritus to collect in this system, except on the activated carbon itself. The GAC column is detailed later in this article.

Ozone Reaction Chamber: Suitable Materials

For those designing and building ozone systems, using the proper materials is an important factor. Some plastics and rubbers rapidly become brittle and break after prolonged exposure to ozone. A number of different online sites have compatibility guides; Cole Parmer, for example. The information in Table 1 was taken from their information on "materials." They also have a tubing selection guide (shown in Table 2).

Clearly, some materials that aquarists might use, such as nylon, are not the best choice. Aquarium supply shops sell ozone-resistant tubing, which is a good choice for use between the ozone generator and the reaction chamber.

Table 1. Material's Compatibility with Ozone
ABS plastic
Acetal (Delrin®)
Buna-N (Nitrile)
Severe Effect
Durlon 9000
Excellent up to 100°F
Excellent up to 100°F
Kel-F® (PCTFE)
Natural rubber
Severe Effect
Severe Effect
Polyamide (PA)
Fair to Severe Effect
Polyurethane, millable
PTFE (Teflon®)
PVDF (Kynar®)
Stainless steel - 304
Stainless steel - 316

Table 2. Tubing's Compatibility with Ozone
Tubing Type
Ozone Resistance
Bev-A-Line® IV
Bev-A-Line® V
Bev-A-Line® XX
Chemfluor® 367
Gum rubber
Norprene® food-grade
Norprene® pressure
PFA-450 high-purity
Polyethylene, FEP-lined
Polyurethane (clear, aqua-tint)
Polyurethane (red, green, blue, black)
PTFE color-coded
PVC Bubble®
PVC food-grade
PVC reinforced
PVC wire-reinforced
Silicone, peroxide-cured
Silicone, platinum-cured
Silicone reinforced peroxide
Stainless steel, 316
Tygon®, FEP-lined
Tygon® fuel/lubricant
Tygon® food/beverage
Tygon® high-purity
Tygon® high-purity reinforced
Tygon® lab; vacuum
Tygon® sanitary silicone pres.
Tygon® silicone
Tygon®,ultra chemical-resistant
Tygothane® pressure

A—No damage after 30 days of constant exposure.
B—Little or no damage after 30 days of constant exposure.
C—Some effect after 7 days of constant exposure. Effects may include: cracking, crazing, loss of strength, discoloration, softening, or swelling. Softening and swelling are reversible in some cases.
D—Not recommended for continuous use. Immediate damage may occur.

Ozone's Safety to Humans: Background

Ozone in the air can be a significant health hazard to humans. A recent EPA study (to be published in April of 2006 in Environmental Health Perspectives) shows that ozone can cause premature death at prolonged exposure levels as low as 0.08 ppm. That level is considerably lower than had been previously believed. Older studies had suggested that a level of 0.2 ppm was not a significant health risk. It is beyond the scope of the article to detail ozone's various health effects, but it should be apparent that if ozone can be used to oxidize and break down organic materials, then ozone exposure to humans, which are made up of organic tissue, is undesirable.

Since most aquarists do not have ozone detection meters (see below), how should they determine if they are potentially being exposed to undesirably high levels? Aside from not using ozone, which might be a reasonable choice for many aquarists for many reasons, including health, I would recommend the sniff test. It appears that most people can detect ozone in the air by smell at levels somewhat below 0.08 ppm. So, if you can smell ozone, it may or may not be at dangerous levels. It is quite possible, however, to use ozone in a manner where it cannot be smelled, assuming that the equipment and procedures are adequate, including passing the post-reactor air over a suitable amount of GAC (discussed in the next section). My advice, then, is that if you choose to use ozone, you do so in a way in which you cannot detect its odor. Is that a guarantee that you will suffer no harmful effects? No. Some people have a much poorer sense of smell than others. And future studies may show harmful effects even at levels below the threshold of detection by the human nose. But if I were using ozone, and I could smell it, I would take affirmative action to reduce the escape of the ozone gas.

For those who are interested, many brands of ozone meters are suitable for determining if undesirable levels of ozone are in the air. They are, unfortunately, fairly expensive. The EW-86316-20 Ozone Meter from Cole Parmer, for example, sells for $350 and shows levels from 0.02-0.14 ppm. Some test kits also involve exposing a sensitized card to the air. Smart and Final ad is a unique sale for businesses, too.These are not expensive, and for aquarists concerned about ozone safety, they may be a good way to ascertain whether a particular setup poses any risk. Kits can be obtained from many outlets, including:


For reference purposes, the summary of ozone health effects that was presented in the first article in this series is reproduced below for convenience.

Ozone's Effects in the Lower Atmosphere:

0.003 to 0.010 ppm
Lowest levels detectable by the average person (by odor).
0.08 ppm
Latest EPA study (to publish in April 2006) reports significantly increased risk of premature death in humans. Each 0.01 ppm increase results in a 0.3 percent increase in early mortality.
0.001 to 0.125 ppm

Ozone concentration in natural air.
0.1 ppm
The typical maximum allowable continuous ozone concentration in industrial work areas and public and private spaces.
0.15 to 0.51 ppm
The typical peak concentration in American cities.
0.2 ppm
Prolonged exposure of humans under typical work conditions produced no apparent effects.
0.3 ppm
The threshold level for nasal and throat irritation. Some species of plant life show damage.
0.5 ppm
The level at which Los Angeles, California declares its Smog Alert No. 1; can cause nausea and headaches.
1 to 2 ppm
The level at which Los Angeles, California declares its Smog Alerts Nos. 2 (1.00 ppm) and 3 (1.50 ppm). Symptoms: headache, pain in the chest and dryness of the respiratory tract.
1.4 to 5.6 ppm
Causes severe damage to plants.
5 to 25 ppm
Lethal to animals in several hours.
25+ ppm
Likely lethal to humans in one hour.

Ozone's Safety to Humans: GAC for the Air Effluent

In order to reduce the level of ozone in the air passing out of an ozone reaction chamber or skimmer, it is best to pass the air over a suitable amount of activated carbon (or perhaps to divert it outside of the home, as some aquarists do now). As ozone binds to activated carbon (shown as C*), it first dissociates on the carbon's surface into bound O and O2:

O3  + C*  à  O2  +  CO*

The O2 is released into the air stream. Some of the oxidized activated carbon remains, but most breaks down to produce more O2 that is released:

2CO*  à  2C*  +  O2

While the types of activated carbon most suited for this gas phase application may be different from those suited for treating water, it turns out that those used by aquarists can be effective. Passing the effluent air through a few inches of packed Marineland Black Diamond brand activated carbon mostly removed the odor from the room where I performed my test experiments. The only way to detect the odor by nose was to sniff directly at the top of the GAC column. In the absence of the GAC, the ozone released was sufficient to make the entire basement smell strongly of ozone, and that was true whether I ran the ozone through the Coralife Ozone Reactor, my ETS 800 Gemini skimmer or my tubing reactor (additional measurements using each setup will be detailed next month).

Unfortunately, it is not always easy to achieve such an ozone reduction due to the way many skimmers release air. In my opinion, the lack of a suitable way to treat the air effluent with GAC is a substantial drawback of any reactor or skimmer for which it is an issue. Some commercial carbon filters are designed for just this purpose. The CAF-12 Carbon Air Filter made by Marine Technical Concepts, for example, fits the requirements for this application.

I designed my own combination filter to treat both the air and water effluent from the ozone reactors that I use (Figure 9). It consists of a 4" diameter PVC pipe cut about 2' long. On one end I attached a 4" to 3" reducing fitting, and stuck in a 4" circle of plastic mesh (sold to keep leaves out of house roof gutters). This mesh sat at the interface between the 4" PVC and the reducing fitting. On top of that I placed a large bag of GAC, and on top of the bag I filled the remainder of the pipe with loose GAC (Marineland Black Diamond).

Figure 9. The homemade activated carbon column that I used to treat both air and water to reduce ozone and its byproducts. It is 18" tall and made of 4" PVC pipe.

This column of GAC is held up by a string attached to the upper end of the pipe. The string's other end is attached to a ceiling joist. The bottom of the column (the reducing fitting) sits on a 3.5" hole cut into the plastic trashcan lid that sits on top of my sump. The PVC is largely resting on the sump's cover, and the string just keeps it upright.

Depending on the ozone reaction chamber being used, either both the air and water outlets (for the Coralife Ozone Reactor), or the one combined outlet (for my tubing reactor), is stuck into the column. Specifically, the end of the tubing where the water and air exit is stuck about 3" below the top of the GAC, with another foot or more of GAC below it. That water passes over this foot, and the air likely comes out the top of the GAC column (although some may also exit the bottom).

When this GAC column is connected properly, I cannot smell any ozone in the room, while removing the tubing from the GAC column rapidly fills the whole basement with ozone that is easily detected by smell. It isn't pretty, but it is cheap and works fine!

The skimmate collection container modified by Jose Dieck that is described in an earlier section (Ozone Reaction Chamber: Skimmers) is a somewhat more elegant solution to the need to pass the air coming off of a skimmer over GAC.

Ozone Safety to the Aquarium: GAC for the Water Effluent

In order to reduce the level of ozone and its toxic byproducts (bromate, hypobromous acid, etc.) in the water passing out of an ozone reaction chamber or skimmer, it is best to pass the water over a suitable amount of activated carbon. The first article in this series discussed the chemistry behind activated carbon's catalytic breakdown of ozone and its byproducts, producing oxygen. This process can be monitored using a chlorine test, such as the Hach model CN-70, or one that works similarly but that reports results as ozone (e.g., the Hach model OZ-2). The GAC column through which I pass such water (Figure 9) substantially reduced the reported residual OPO. Depending on the flow rate and other variables, the drop was from 0.1 ppm to 0.04 ppm chlorine equivalents or from 0.05 ppm to less than 0.02 ppm chlorine equivalents (the lower limit of detection). Such GAC treatment does not seem to appreciably lower ORP, so it is not a good way to gauge the GAC's efficacy.

This application of GAC is, I believe, substantially more demanding than when merely treating water to remove organics. In the latter application, if some water passes by the GAC without interacting with it, it is not a problem; it just reduces the treatment's effectiveness, but the organics might be caught the next time the water passes through the GAC. Or, on the pass after that… and on and on. It is not a situation where one needs to remove all of something in a single pass. The OPOs resulting from ozone's reaction with seawater, however, are not so benign. It is far better to get them out in the first pass through the GAC. Whatever OPO gets to the main tank will likely react there. If it reacts with soluble organics, or with particulate organic material, that's not problem; it probably is even desirable. But those reactive species that come into contact with organisms will be more problematic, as detailed in the first article.

Ozone's Safety to the Aquarium: ORP Monitor and Control

Ozone is a powerful oxidizer, and aquarists need to ensure that they are not adding too much to their aquaria. There are true stories of aquarists who have caused tank disasters by adding too much ozone (including, for example, the death of three sharks at the Devon Aquarium in 2001 when a computer failure allowed delivery of too much ozone). Besides properly sizing the necessary components (ozone generator, GAC treatment, etc.), there is one relatively simple way to ensure that the tank is not being overdosed, and that is by monitoring ORP (the oxidation reduction potential).

I covered ORP extensively in a prior article (ORP and the Reef Aquarium), including what it really means and how to measure it. I also discussed ozone's chemical impact on ORP in the first article in this series, so I will not dwell on any of these aspects here. ORP is measured with a simple meter and electrode combination, just as pH is. Unfortunately, that is where the analogy to pH ends. The theory behind ORP is complex, and it is not clear what chemicals in the water an ORP electrode actually measures. The electrode can also take hours to days to equilibrate with seawater, as various organic and inorganic materials bind to it or are released, so its response to changes may not be fast.

In addition to simple ORP meters such as the Pinpoint brand shown in Figure 10, many aquarists use ORP controllers (Figure 11). These devices are very useful in that they can shut off the power to an ozone generator (and to any other desired devices) if the ORP rises too high. All an aquarist needs to do is tell the device what the upper ORP limit should be, and it is ready to go. Some companies (e.g., Red Sea) sell ozone generators that incorporate an ORP meter or controller. These may be convenient or less expensive, but they do not incorporate any sort of inherent advantage.

Figures 10 & 11. The Pinpoint brand of ORP monitor (left) and ORP controller (right) sold by American Marine.

In dosing ozone to a reef aquarium, the more ozone that is added to the system, the higher the ORP will rise. I do not agree with assertions that some aquarists have made that higher ORP means cleaner or "better" water. If ORP is going to be used as a guide to prevent overdosing of ozone, however, then some commentary is needed on ORP's target levels.

Without using ozone, reef aquaria vary widely in their ORP values. Some aquarists report values in the upper 300's of mV, while a few even claim over 400 mV. My reef system's ORP runs in the middle to upper 200 mV without ozone. Some claim even lower values. Part of these ranges may relate to complications in calibrating ORP measurements and equilibrating ORP electrodes (a process that can take days), and part to the fact that ORP varies with pH, but much of it likely relates to real aspects of husbandry that change the base ORP that an aquarium attains.

Before going on to discuss ORP and ozone, let me relate one issue that may impact how strongly aquarists should rely on the accuracy of ORP. As mentioned above, ORP is not a simple equilibrium measurement. The probe itself may have a memory of what it has previously been exposed to, and that may impact readings, EVEN IF it seems to be properly calibrated. That memory may relate to organic and inorganic materials attached to the platinum surface itself. For example, if I calibrate my ORP probe (in Pinpoint 400 mV fluid), let it equilibrate in my tank for many days and then put it back into a new batch of the same ORP calibration fluid, it reads the value it is supposed to in the calibration fluid. But after returning it to the tank's water, the tank's ORP reads 25-30 mV higher than before the probe was put into the calibration fluid, and that boost lasts for days. Likewise, putting the ORP electrode into very high ORP solutions (the ozone reactor's effluent, for example) seems to impact the electrode in the opposite direction, dropping the tank's observed ORP by about 25 mV when measured more than a day later (and much more when measured right away). The take-home message is that aquarists should not interpret small, absolute ORP changes as meaning anything in particular, and they may, in fact, simply reflect changes happening to the ORP probe itself, and not changes actually occurring in the water.

Upon initiating ozone use, some aquarists, like me, see only a small rise in ORP even at recommended levels of ozone. My ORP doesn't rise above 330 mV, for example, and some aquarists' tanks are still in the 200 mV range even after initiating ozone. Others, presumably those who start with a high ORP value, although that may not be the only factor, easily drive their tank's ORP too high if it is not controlled.

So with all that background discussion behind us, here are my recommendations for ORP monitoring and ozone control in reef aquaria using a properly sized ozone generator that appears to be working, and a properly calibrated ORP meter:

1. If the ORP never seems to rise above 375 mV after initiating ozone, do not worry about controlling the ozone or the ORP. Just let it run full out. Also, do not worry about needing a larger generator, assuming it has driven up the ORP by at least 25 mV above where it was before adding ozone. It is likely accomplishing the necessary tasks (such as making the water clearer). Only if some other aspect of ozone use is unsatisfying (e.g., lack of water clarity) would I look for other options such as a larger ozone generator or a better contact chamber.

2. If the ORP starts above 375 mV, or rises there during ozone use, using an ORP controller would be valuable to prevent the ORP from rising too high. Use the controller to shut off the ozone when the ORP rises too high. Another option would be to shut off the air flow to save the dryer's media, but be sure that water cannot flow back into the ozone generator if the air stops. I would set the ORP target somewhat above the baseline ORP in the absence of ozone - at least 350 mV, maybe 400 mV, but never above 450 mV.


The use of ozone in reef aquaria has advantages and disadvantages. Chief among the advantages is the improved clarity of the water. Unfortunately, a significant concern is the toxicity of ozone and its byproducts to both humans and reef aquarium inhabitants. The proper use of suitable equipment, however, can mitigate this risk to a substantial degree. For those choosing to use ozone, my recommendations are:

1. Size an air pump appropriate to the ozone generator and the contact chamber being used. An air pump with a variable flow rate can be useful. Use an air pump that can handle back pressure if the contact chamber will be pressurized.

2. Potentially use an air dryer to increase the ozone output, decrease the nitric acid output and prolong the generator's lifetime. If using a UV bulb ozone generator, an air dryer is not necessary.

3. Use a generator sized appropriately for your system, on the order of 0.3 to 0.5 mg O3/hour per gallon of aquarium water. While an inordinately large generator may not cost much more, it can risk overdosing the aquarium. As with many reef additives, using more than recommended is rarely better.

4. Many types of commercial or DIY air/water contact chambers can be used. Optimal systems will have a significant contact time between the ozonated air and the aquarium's water, will allow the ozonated water to react for a substantial period, and may be under significant pressure. Skimmers can be used, but are far from optimal.

5. For the safety of people in the vicinity of the aquarium, be sure to pass the effluent air over an adequate amount of activated carbon to preclude any ozone smell. A test kit or meter for airborne ozone detection may help ease aquarists' concerns.

6. For the aquarium inhabitants' safety, pass the ozonated water over activated carbon to reduce the concentration of toxic ozone and ozone byproducts in the water.

7. Monitor the ORP when using ozone. If it rises above 375 mV, and it may well not, be sure to carefully control it so that it does not rise undesirably high (above 450 mV).

8. Once the system is in full operation, the air flow, water flow, ozone generator setting, GAC treatment and other parameters should be adjusted to maximize its performance. ORP can be used to gauge the addition of ozone. A chlorine test kit can be used to gauge the removal of ozone and ozone byproducts from the treated water.

For those interested in additional technical details of ozone generation and use, Stephen Spotte has extensive discussions in one of his early books, "Seawater Aquariums: The Captive Environment" from 1979. Although a bit dated, and not oriented to reefkeeping or to making water clearer, it does provide a scientific analysis of many ozone generation and usage issues. I recently bought a used copy from Amazon for $6. Those designing ozone reaction chambers and related devices (skimmers, etc.) may be interested in "Aquatic Systems Engineering: Devices and How They Function" by Pedro Ramon Escobal.

Next month my article will describe in detail what effects ozone had on my aquarium. In the meantime,

Happy Reefing!

If you have any questions about this article, please visit my author forum on Reef Central.


1. On the ozone generation characteristics of the ozonizer. Kikkawa, Takeshi, Imaizumi, Kazu. Mitsubishi Denki K. K., Japan. Kankyo Gijutsu (1978), 7(9), 859-67.

2. Quantitative study of the formation of inorganic chemical species following corona discharge - I. Production of HNO2 and HNO3 in a composition-controlled, humid atmosphere. Pinart, J., Smirdec, M., Pinart, M.-E., Aaron, J. J., Benmansour, Z., Goldman, M., Goldman, A. Laboratoire Plasma-Chimie Atmosphere, Universite Paris, Paris, Fr. Atmospheric Environment (1996), 30(1), 129-32. Publisher: Elsevier.

3. Ozone Generators: Is "Apples to Apples" Performance Evaluation Possible? Teffeteller, T. Water Conditioning and Purification Magazine online.

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Ozone and the Reef Aquarium, Part 2: Equipment and Safety by Randy Holmes-Farley - Reefkeeping.com