Calcium carbonate reactors have become a popular way of replacing the calcium and carbonate taken up by corals in the process of calcification.

In its most basic form, a calcium reactor is simply a container filled with calcium carbonate (CaCO3) media over which aquarium water is passed with the addition of carbon dioxide. Adding carbon dioxide lowers the pH of the water, making it acidic, and dissolving the calcium carbonate to provide the aquarium with calcium and alkalinity.

Many different designs of calcium reactor are now available, but it is not the purpose of this article to suggest or review any particular model. Instead, I would like to concentrate on the subject of setting-up and using a calcium reactor. For simplicity, I have avoided using chemical equations, and suggest interested readers refer to the "further reading" section at the end of the article.

Setting Up the Calcium Reactor

The first step is to assemble the calcium reactor. Because each model is different, the user should refer to the manufacturer's instructions supplied with the reactor. Some of the common parts associated with a calcium reactor are described below:

Calcium Carbonate (CaCO3) Media

Most calcium reactors are not supplied with calcium carbonate media. Unfortunately, choosing good media is not easy as there is very little information published on the composition or impurities present (Bingman 1997, Hiller 2001).

An important thing to bear in mind is the pH level you will need to achieve within the calcium reactor to dissolve the medium. In a typical reef tank with pH 8.2, the calcium carbonate is supersaturated, and it tends to precipitate onto other fresh calcium carbonate surfaces. At typical reef tank calcium and alkalinity levels, a pH of around 7.7 or less is needed inside the calcium reactor for aragonitic media to begin to dissolve (Holmes-Farley 2002). Generally, most people get good results dissolving aragonitic media inside the reactor at pH 6.5 to 6.7, but be aware that some of the harder CaCO3 media, such as those made of calcite, will require an even lower pH to dissolve easily. Dropping the pH too low inside the reactor (in my experience, this is less than 6.5 for aragonite) often leads to the media turning into fine particles that slow the water flow through the reactor.

Carbon Dioxide (CO2)

Because CO2 is required and supplied in a pressurized container, having a CO2 bottle in your living space requires you to observe a few safety precautions!

Ensure the CO2 bottle is regularly checked when refilled to make sure there is no loss of structural integrity. The company filling your bottle can perform this inspection.
Fasten the CO2 bottle securely with a safety cage or straps when in use, so it cannot be accidentally knocked over. If the bottle is knocked over and the collar broken off, it can take off like a rocket!
Remember CO2 is colorless and odorless, and acts as an asphyxiant. Always open windows before working near the bottle if you suspect a leak.
High temperatures can cause CO2 bottles to explode! Do not place the cylinder near a source of high heat, such as a heat radiator.

CO2 bottle and regulating equipment. Photo courtesy of Skip Attix.

Attached to the CO2 bottle is a regulator consisting of the valves and gauges used for controlling and monitoring the rate at which CO2 is released from the bottle. Most regulators have two gauges, one showing the bottle pressure and the other, the operating pressure.

The needle valve, the most critical part of the regulator, is used to make fine adjustments to the CO2 bubble rate. A working pressure of 15 psi (1 bar) is often necessary to ensure the bubble count remains steady. If you are using a solenoid valve (see below), please check with the manufacturer of the device to determine what pressure it is capable of withstanding.

Users often complain that the adjustment of the valves is rather coarse, with a fraction of a turn resulting in a steady bubble rate likely turning into a continuous flow. A solution is to buy a higher quality inline needle valve capable of precise adjustment, and fit it on the tubing between the CO2 bottle and the reactor.

Solenoid Valve / pH Control

A solenoid is simply an electrically operated valve. When electricity is supplied to the valve, it opens, and when the electricity is off, it closes. Its possible uses are as follows:

The simplest and most common way to utilize a solenoid is to plumb it into the line between the CO2 bottle and the calcium reactor. In the event of a power outage, the CO2 flow is switched off, stopping any gas escaping from the calcium reactor into the tank.

A more elaborate method is to connect the solenoid valve to a pH controller and place the pH probe into the calcium reactor. The valve then switches the CO2 on and off to maintain a target pH within the reactor. I use a similar method. The pH probe is placed in the tank, and switches off the CO2 flow to the calcium reactor only when the pH in the tank has dropped too low (e.g. pH 7.8 or lower).

Feed Pump

There are a number of ways to supply the calcium reactor with water from the tank. Some reactors/methods siphon water into the suction side of the calcium reactor's re-circulation pump. I have used this method, but I found it unreliable because the medium in the reactor starts to dissolve and compacts, putting more and more back pressure onto the pump, and resulting in less suction and, therefore, less water into the reactor. To prevent this from occurring, most aquarists prefer to supply water to the reactor either using a 'T' fitting from their sump return pump, or a small power head fitted with a ball valve to adjust the flow. This technique may work well, but can be difficult to adjust properly as ball valves have a very small 90-degree turn from completely off to fully on. A gate valve or needle valve is a better adjustment device, but occasionally the valve becomes clogged with debris and needs cleaning. By placing the valve on the outlet side of the reactor you will achieve a more stable flow than trying to control it from the inlet side.

Personally, I use a peristaltic pump to supply water to the reactor. Peristaltic pumps are very good at operating against pressure, providing a steady flow with the minimal maintenance requirement of replacing the tubing once in awhile. A simple rotary device controls the motor's speed, allowing easy and very precise adjustment of the flow even at low flow rates. I recommend using a high quality unit that is specifically designed for a 24 hr./7 days a week duty cycle. Most pumps sold for the aquarium hobby are not suitable! (Watson-Marlow pumps have been found to be very robust for this job; one aquarist I know has run one continuously for over 7 years!)

Calcium reactor with secondary de-gassing chamber. Photo courtesy of John Link.
*Blue pipe – water from aquarium.
*Yellow pipe – CO2.
*Green pipe – effluent from reactor.

Tuning the Reactor

Once the calcium reactor is assembled, the next step is to tune it to meet the calcium and alkalinity demands of the tank. There are several different ways to tune the reactor, but I will describe the method that I (and many other reef-keepers) use.

IMPORTANT: As with all things in reefkeeping, it is important to be patient! After making adjustments to the reactor, it should be left for a few hours to allow the changes to take effect. Resist all temptation to meddle and tinker with the settings un-necessarily.

Two controls are used to adjust a calcium reactor. One controls the effluent, or the amount of water flowing through the reactor, and the other controls the amount of CO2 added to the reactor, usually measured by the number of bubbles of CO2 in the bubble counter.

The following steps describe the tuning process:

Step 1)
Set the reactor at a fairly low CO2 bubble count and a low effluent flow rate. Most manufacturers suggest guidelines, which for my reactor was 40 drips per minute of effluent water and 10 bubbles per minute of CO2.

Step 2)
Then adjust the pH within the reactor to approximately pH 6.5 to 6.7 for dissolving the medium. First, measure the pH of the effluent exiting the reactor with a test kit or pH probe (I recommend a pH meter as most pH test kits are not sufficiently accurate). If the pH is too high, reduce the effluent flow rate; if the pH is too low, increase it. Allow a few hours for the reactor to respond to the changes, and repeat this step until the pH value is between 6.5 and 6.7.

Step 3)
Monitor the tank alkalinity level to ensure that the reactor is supplying enough calcium carbonate to replace that being used by the animals in the tank. An alkalinity test kit may be used to measure these levels (1 mEq/L change in alkalinity is only 20ppm calcium!). For future reference, it is a good idea to keep a logbook of the tank's alkalinity level and any adjustments you have made to it.

Measure and record alkalinity every few days and compare the readings. If the alkalinity level is falling, increase the amount of CO2 so more of the medium is dissolved. Conversely, if the alkalinity level is rising above the level you want, reduce the amount of CO2 so less of the medium is dissolved.

Of course, making adjustments to the CO2 rate will affect the pH level inside the reactor. A quick fix to keep the pH stable is to make the same adjustment to the effluent flow rate as you make to the CO2. For example, if you double the CO2 rate, double the effluent rate, too; this is only a rule of thumb, but should prove effective.

When finished, double-check the effluent to verify that it is still around pH 6.5. If not, you can repeat step 2.

Step 4)
After the reactor is set up, check the tank alkalinity levels periodically for a few weeks to take into account the calcium carbonate requirements of any new additions and coral growth in tank. Also, as the medium becomes depleted you may need to re-adjust the reactor, or refill it. If adjustments are required, simply fine-tune the reactor using the steps outlined above.


Low Tank pH

After adding a calcium reactor, many aquarists complain that the pH of the tank is lower than it was previously. Aquarists often think that excess CO2 in the effluent that has not had time to react with and dissolve the media is the reason for the reduced pH. However, remember that the calcium reactor is adding alkalinity, mainly in the form of bicarbonate, (which itself will depress the tank pH) until excess CO2 is degassed into the atmosphere. Some of the bicarbonate is then converted into carbonate. This is very similar to the effect observed when adding sodium bicarbonate to your tank as a buffer.

In order to rid the tank of any excess CO2 and maintain a good pH, it is essential to have good circulation at the air/water interface.

The pH can also be boosted by using limewater as top-off water. Limewater (also known as kalkwasser) works by using the CO2 in the tank water and the hydroxide ions from the limewater to increase the alkalinity. In turn, removal of the excess CO2 leads to an increase of the tank pH.

Another popular technique to remove excess CO2 is to degas the effluent, either by running it through an additional container of calcium carbonate chippings or by dripping the effluent into a small container housing an air stone. Results from these methods vary, with some aquarists reporting significant increases in alkalinity or pH and others seeing little observable difference (probably due to different calcium reactor designs and their effectiveness). With both of these methods you must be careful. As the pH is driven back up towards natural seawater levels, some of the bicarbonate is converted into carbonate. Once the water becomes supersaturated with carbonate, it will be more inclined to precipitate onto calcium carbonate surfaces, and some alkalinity will be lost.

Out of Balance

Another common problem when setting up a calcium reactor is getting a correct balance between calcium and alkalinity. A common complaint is as follows:

"I have an alkalinity of 3.5 mEq/L (10 dKH), but my calcium level is only 320ppm. I have tried adjusting the reactor, but cannot get the calcium level to rise without the alkalinity going too high."

A calcium reactor may be described as a 'balanced' calcium / alkalinity additive. Basically, this means that it adds calcium and alkalinity to the tank in the same ratio as is used by our corals during the process of calcification. Simply put, it is not possible to change the calcium level without the alkalinity being affected also in a defined manner.

As an example, for each 1 mEq/L alkalinity (2.8 dKH) the calcium reactor adds 20ppm calcium. If your tank starts out with 3 mEq/L alkalinity (8.4 dKH) and 320 ppm calcium, and you raise the alkalinity to 4 mEq (11.2 dKH) using the calcium reactor, then the calcium level will only increase to 340 ppm!

Natural seawater at 35 ppt salinity typically has around 2.5 mEq/L alkalinity (7 dKH) and a calcium level of 410 ppm, but I personally aim for around 3 mEq/L alkalinity (8.4 dKH) and 420 ppm calcium, and many others prefer even higher levels. Once you have decided on the levels, it is a useful idea to map where the calcium and alkalinity levels are (Bingman 1998) and then perform any corrections needed to get them back on target.

If the calcium level needs boosting, then I recommend using an additive such as calcium chloride. One gram of an anhydrous calcium chloride product (such as Turbo Calcium) will raise the calcium level by 360 ppm in 1 litre of water (95 ppm in 1 gallon of water).

If the alkalinity level needs boosting, then sodium bicarbonate can be used. One gram will raise the alkalinity by 12 mEq/L (34 dKH) in 1 litre of water (3.2 mEq/L (9 dKH) in 1 gallon of water).

In both cases, I recommend making changes slowly, rather than adding them all at once.

It is also worth noting that you may have difficulty achieving natural calcium and alkalinity levels if your salinity is less than natural seawater (35ppt) (Holmes-Farley 1998) or if you have a deficiency in magnesium (Bingman 1999, Holmes-Farley 2001). A solution to magnesium depletion, used by some aquarists, is to include a few teaspoons of pure dolomite in the calcium reactor where it can dissolve, adding magnesium to the tank (Bingman 1997).


Too often equipment is not supplied with detailed instructions to guide the new user through the complex maze of fine-tuning a calcium reactor. I hope this article has provided a better understanding of the principles, equipment, and operation of a calcium reactor.

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


Bingman, C. 1997. Calcium carbonate for CaCO3/CO2 reactors: more than meets the eye. Aquarium Frontiers. August, 1997.

Bingman, C. 1998. More about calcium and alkalinity. Aquarium Frontiers. July, 1998.

Bingman C. 1999. Magnesium - Part II. Aquarium Frontiers. April, 1999.

Hiller, G. 2001. Alternative calcium reactor substrates. Aquarium Frontiers. 2001.

Holmes-Farley R 1998. Understanding seawater. Aquarium Frontiers. July, 1998.

Holmes-Farley R. 2001. Magnesium: calcium's little sister. Aquarium Frontiers. 2001.

Holmes-Farley. R. 2002. Calcium and alkalinity. Reefkeeping 1(3).

Further Reading:

Carbon dioxide: friend or foe? Aquarium Frontiers. 2001.

Building your own calcium carbonate reactor. Aquarium Frontiers. March, 1998.

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A Guide to Using Calcium Reactors by Simon Huntington -