Reefkeeping 101 -
Ho! Ho! Ho! - Let’s Light 'em Up!
One of the areas that many new reefkeepers find confusing, and expensive, is lighting a marine tank. If you have a freshwater tank, you probably use mood lighting; that is, lights that you find pleasing. Oh, you may use a grow lamp or full spectrum aquarium light if you have plants, but otherwise it is mainly whatever you like as far as lighting is concerned.
This is fine if your marine tank will be a fish-only (FO) system because saltwater fish are no more demanding in terms of light intensity than their freshwater cousins. I do avoid lava lamps with marine fish, however, because they could get scared if they think an undersea volcanic vent has opened. On the other hand, if you intend to have a full-blown reef tank, then lighting is far more important. Using mood lighting here is not an option, but if you choose the proper combination, you can find one that keeps both you and your tank’s inhabitants happy.
The reason for the difference between fish and coral systems has to do with corals’ symbiotic dinoflagellate algae. Known as zooxanthellae, these are important to corals, anemones and some other critters such as clams that depend on algae to provide nutrition. These algae live in the tissue of corals and other organisms. Safe and protected from harm, they return the favor by producing excess food during photosynthesis, which the coral can consume. In some species of stony corals they provide over 80% of the nutrition that the coral needs to survive. Once fed, the corals return the favor and produce waste products rich in carbon dioxide, nitrogen and phosphorus that the algae need to survive.
In order for these buried algae to photosynthesize, they need a far more powerful light than would be needed for unsheltered, low-light algae such as coralline. This light must penetrate through both the water and the corals’ tissue while providing the proper intensity and spectrum for photosynthesis to take place. This is what makes lighting a reef tank a challenge. Photosynthesis is a key component for maintaining a true reef tank.
How the Bulbs Work
Using incandescent bulbs to light a tank is pretty much passé, and even freshwater tanks are illuminated with fluorescent tubes. Fluorescent tubes, albeit on steroids, are also used in marine tanks. Knowing how a fluorescent light works helps us to selecting the proper ones for a marine system. Basically, it’s a glass tube with two electrodes - one at each end. The tube is filled with an inert gas and contains a small amount of mercury. To light the lamp a high voltage is applied, and some of the mercury vaporizes and makes the tube’s interior conductive. A current passes down the tube to the other electrode and the mercury starts to glow. This is all fine and dandy, but it isn’t much of a glow, and most of it is invisible ultraviolet light at that. To get the tubes to work, the glass tube is coated with a material known as phosphor. The energy from the glowing mercury cloud strikes the phosphor and all sorts of reactions takes place with the phosphor’s atoms. Normally the phosphor's electrons are at rest and orbit in a spherical cloud, called “s configuration.” The mercury radiation imparts energy to the electrons, and they get so excited that they change the cloud’s shape to a solid figure eight pattern called a “p configuration.” However, those lazy phosphor atoms don’t really like being souped up, and an instant later they drop back to their spherical rest state. When they do, they must shed the energy they absorbed, and they do so by emitting photons of light; that is, they fluoresce. Now the real magic here is that they don’t need to emit light at the same spectral level as the mercury; they can change the wavelength to one that's more visible to the human eye. This is how all fluorescent tubes create useful light.
When fluorescent tubes first came onto the market they were somewhat limited in the wavelength they produced. The public at that time was accustomed to the somewhat pinkish hue of incandescent lighting, so the light manufacturers chose phosphor that somewhat duplicated it. Called “warm white,” it had a slightly pinkish tinge. Later, to give the light more balanced tones, “cool whites” were created with less pink in their discharge. Over the years the phosphors used have become more and more complex, and now it is possible to have almost any color of the rainbow in a fluorescent tube. The common ones in the marine aquarium trade are usually tri-phosphor coated. Phosphors are used that produce red, green and blue light that can be combined to make different shades of white. This is like a color television where europium-doped yttrium phosphors create bright red, zinc-copper aluminum for green and zinc-silver cobalt blue, when struck by the TV’s electron beam.
Now a little something is also happening in that mercury cloud in the tube. As I said, a high voltage surge is supplied to the tube to get things started, but once the tube fires, it starts to warm and more and more mercury is vaporized. The resistance inside this mercury cloud starts to drop like a rock, so, without a solution, the lamp simply would get brighter and brighter until it burned out. Fortunately, there is a solution. All fluorescent tubes employ a ballast. A ballast performs two functions. First, it uses a capacitor to provide the initial high voltage needed to light the tube. Once the tube lights, the capacitor then acts as a transformer that limits the amount of current that the lamp consumes, and prevents it from winking out the first time it is turned on.
Ballasts in the good old days, and some that are still used widely today, are common magnetic ballasts. They resemble a small brick and are fairly cheap. They are, however, specific in what they can power. The tube and the ballast must be matched or the system will not work. Replacing the magnetic ballasts in many applications is a new breed of electronic ballasts. These are smaller, run cooler and usually can handle multiple types of tubes. A few years ago I favored magnetic over electronic ballasts because the latter were more expensive and their mean time between failures was about half a day. The industry has solved this problem, and electronics now can last as long as magnetics, which is 10 years or more. They also have reduced radio frequency emissions. Magnetic ballasts produce heat, but electronics produce radio noise. The current crop has reduced the amount generated and shielded what remains. Prices have also fallen. If you plan to upgrade your tank over time, then buying a somewhat oversized electronic ballast is probably a wise choice.
Let's take a look at metal halide (MH) lights and how they work. They have some things in common with fluorescents. They contain an inert fill gas and usually a small amount of mercury to get them started. They also require a ballast that does the same jobs as the fluorescents, that is, starting the lamp and limiting the current. However, MH lamps cannot run on a fluorescent ballast and fluorescents cannot run on a MH ballast.
The difference lies in how they produce light. The filaments in a MH lamp are close together. When current passes through them, they heat fairly quickly to very high temperatures. Inside the tube are also metals ions combined with, in most cases, iodine or bromine. As the arc's temperature increases, these ions become volatile and enter the gas cloud that is being heated between the filaments into a very hot state known as plasma. Unlike phosphors, which absorb energy and then release light when they relax, plasma creates great excitation in the metals and emits photons at high energy levels. By using various combinations of halide metals, these bulbs produce various parts of the spectrum that we see as visible light.
Which to Choose?
I had planned to have that well-known talk show host, Jerry Springer, to help us select which type of lights to choose. Unfortunately, he is on the road giving lectures on how to reduce violence in T.V. programming. However, he did send what was to be his closing statement for the topic, and I’ll present it here:
“Countless families are torn apart over the topic of choosing the best lighting for a reef tank. Very often the family members fail to discuss the matter before selecting the lights, and someone is always dissatisfied. This could be avoided if they talked the matter out to reach a civil meeting of minds. It is never a happy family when one person so dominates that they fail to see the needs of others in the household. Anger mounts further when other household members come in after a long night of collecting aluminum cans from the street so they can help pay for those new metal halide lamps you bought. When this does happen the family should contact my producers, who may invite them to beat the heck out of each other in front of a live studio audience.”
Often on Reef Central the topic of lighting gets pretty heated. Some swear that you cannot maintain a proper marine tank without metal halide tubes; others swear by fluorescent lamps, but often disagree over what type to use. Some like power compacts; others favor common T-12 Very High Output (VHO) tubes. In order to make sense of all this, we need to know a little bit more about light itself.
When shopping for light bulbs you almost always see marked on the packaging, “Bulb X delivers 3600 lumens.” What the heck is a lumen? The lumen is a standard of light intensity that equates light output to wattage supplied. One watt of energy is equivalent to 683 standard lumens. But, there is a catch. The committee that oversees international standards chose Clark W. Griswold of Chicago, Illinois to be the Standard Observer for the lumen. They found that Clark saw light best at a wavelength of precisely 555 nm, so they defined the standard lumen at that level. I was going to get Clark to comment on why this was, but every time I tried to contact him, I was informed that he was on vacation.
Now this seems just fine until we realize that something is amiss. A 40-watt lamp should put out 40 × 683, or 27,320, lumens. That Sylvania 40-watt daylight fluorescent claims to put out only a measly 3600, according to its package. What’s the story? Well, the value of 683 lumens per watt is based on a theoretical comparison. That output exists only in the world of science, and is not all that germane to the real world. I recently compared daylight lamps and found that a Sylvania T-12 fluorescent tube claimed 90 lumens per watt, a T-5 fluorescent got about 85 and a metal halide got about 88. All these values were in the range of only 12-13% of what their theoretical output should be. That is because electrical wattage is not transformed into emitted light very efficiently. Much of the juice we feed into our lamps ends up as heat. A metal halide has a gas plasma that is running close to 7000°C, but what about the fluorescents? They are cool, aren’t they? Not really. A metal halide concentrates all its light and heat in a small area, but a fluorescent spreads its light and, for that matter, heat, down a long tube. It feels only slightly warm to the touch, but if we add all the heat emitted over the bulb's entire length it is close to what the metal halide gives off in its limited arc area. Something else: when the phosphor in the fluorescent is hit by the U.V. radiation, it emits photons in all directions. That means that many of the photons go right back into the center of the tube, or strike the phosphor atoms next to them. Because the light a phosphor emits is not the same as the U.V. light that excited the phosphor in the first place, it is lost in the tube.
Follow me so far? Then we also have our standard observer, Clark. He sees things well only at 555 nanometers, but light may also be emitted at other wavelengths. Here I better stop and explain the nanometer thing. Back in school you all remember the mnemonic “Every Good Boy Does Fine,” don’t you?
“Waterkeeper, for Pete’s sake! That’s the mnemonic of the notes on the score of a page of music.”
Oh, sometimes I get lost; thanks. What I meant was-
These are the colors of the rainbow and the spectrum. Now Clark sees around “G” (green) the best, so the manufactures of light meters, such as those sold for cameras, are weighted to respond in this region, which is around 555 nm. That means if our lamp produces photons outside this region, they will not register as greatly as those in the yellow-green region of the spectrum. With all these things working against our little old fluorescent tube, it's no wonder its actual lumen output is only a little over 10% of what the formula suggests.
Lumens also have another drawback. They are measured at the light source's surface. That is fine to figure a particular light's efficiency, but it doesn’t tell us what happens when the light is transmitted. Get out that old flashlight and turn it on. Place it a foot from a wall and note the size of the circle it illuminates. Now move it back to two feet. The circle has grown, hasn’t it? In fact, it didn’t just double in size; it quadrupled in size. It also appears dimmer. Move it to three feet and the circle is now nine times the size it was at a foot; move it to four feet and it grows to 16 times its original size. Each time it gets dimmer and, if we measured its output, it would be only a ninth as bright at three feet, and a sixteenth as bright at four feet, as it was at one foot. This is because the flashlight bulb produces a fixed amount of lumens and, as the area it illuminates increases, the lumens falling in a given area decrease by the same factor as the distance. Also, if we move the flashlight so that, instead shining perpendicular to the wall, it shines at a 45° angle, we get an egg-shaped oval whose small end is brighter than the wide end.
To measure light traveling over a distance we need another unit, called a lux. A lux is the number of lumens illuminating the surface of a given area, and the standard is based on lumens per square meter. This is more of a real world unit than the lumen because, in our aquarium, the light must travel from the light hood down to the tank’s bottom. In the second article I wrote in this series, I said, “Height is always an issue and I caution novices not to get excited about very tall tanks…A three-foot deep tank requires almost nine times as much illumination to provide the same level of light as in a one-foot deep tank.” This was referring to lumens, where we are measuring light over an increasing distance. The little flashlight experiment helps you to visualize what I meant.
Using lux is helpful but it doesn’t consider the total spectrum; it uses only that yellow-green portion where a lumen is based. In a reef tank the idea is to provide light that promotes photosynthesis, and it just so happens that 555 nm is not optimal. Indeed, some types of chlorophylls like light in the violet region at a little over 400 nm, and others do best toward the yellow-orange region at 575-625 nm. Scientists in recent years have considered this and come up with a new standard, called PAR. This stands for Photosynthetic Active Radiation, and it measures light in the entire visible spectrum responsible for photosynthetic activity. Originally, the meters used for this were known as Quantum meters, and they were very expensive compared to normal light meterd. Now they are available at reasonable prices, and well-equipped aquarists may have one to take these measurements. Better yet, talk your local reef club into buying one. You take these types of measurements only every few months, so the entire club can share a single meter.
So if we know the PAR, we have it made, right? Well, not exactly. PAR is measured over a distance, and it is distance in the reef tank that stirs up much of the debate about which lighting system is best. A fluorescent tube disperses light down a long tube; whereas a metal halide concentrates it in a small area. If you look directly at a metal halide lamp it is almost blindingly bright, but even the brightest fluorescents might just make you squint. Back when I was talking about lumens for different lights, you may have noticed that the number of lumens actually produced by a metal halide and a fluorescent were surprisingly close together. This is because neither system has a marked advantage over the other. What differs is how they dispense those lumens. The halides concentrated light as a point source, and the fluorescents spread it out in a continuum fashion. That means if we have equal wattage in MH and fluorescent tubes, we supply about the same amount of light. It is just that the MH concentrates it, like a flashlight, over a small area, and the fluorescent distributes the light fairly evenly down the whole tank. If used properly, either is fine for illuminating a home aquarium. The issue is how to adjust the light they produce.
If the flashlight you used is like a Maglite, then it has a beam that can be adjusted from a wide to narrow pattern by moving the lens. This is possible because the light is first focused by a reflector, which collects the photons moving in all directions from the light source and reflects them toward the lens opening. In a flashlight the type of reflector is called “parabolic,” which is an elongated hemisphere that allows the light source to be moved to a point where all photons reflect at a single right angle toward the reflector’s opening.
Metal halide lighting acts much like a point source, so parabolic reflectors can be used to focus their beam without the use of a lens. Moving a MH bulb to the proper point in the reflector can create a tight beam, and moving it to another spot can produce a wide beam, much like a Maglite. As I mentioned earlier, the number of total lumens per watt differs very little between a fluorescent light and a metal halide. Because a MH lamp is more of a point source, however, it allows better focusing of the beam, which results in the ability to increase the total lux of the area illuminated. Our elongated fluorescent lamp is cannot be focused in such a manner, so its light covers a broad area at the cost of lux in the wide area illuminated. As a general rule, if you want very high light levels in a fairly small area, always use MH. This also goes for deep tanks. Both fluorescent and MH lights spread the light pattern as the tank's height increases, thus reducing the total illumination at the tank’s bottom. However, because MH lights can project a very tight beam, the effect of the distance increase can be compensated for more easily than is possible with fluorescent lights.
So far it seems that MH is the way to go, but wait a minute. A MH beam is really strong in an area only a little larger than the reflector’s diameter. Outside this area the light's intensity starts falling as fast as a shooting star. Something placed just six inches outside the main beam's area will receive very little light. Fluorescent lamps do not suffer this effect. Their light remains fairly constant along their entire length, so they provide far more even light. Those in the aquarium hobby, especially the lighting suppliers, are well aware of this benefit. The first solution was to get more light from fluorescent tubes. Bulb manufacturers initially doubled the wattage in the tubes without lengthening them, calling them High Output (HO); and later tripled their output and called them Very High Output (VHO). This allowed what at one time was a 40-watt fluorescent to output 110 watts in the same four-foot long tube. This little trick kept fluorescents in step with the metal halide group as a viable type of aquarium lighting.
We need to know a little more about fluorescents at this point. The common household fluorescent bulb has long been the T-12 fluorescent. The number following the T is the tube's diameter in eighths of an inch. This means that a T-12 is 1½" in diameter. That diameter is fine if you are lighting a room, but we don’t really want a bulb that thick in an aquarium. With the shop light type reflectors used in many aquariums, as much light strikes the floor and walls as it does the tank’s bottom. Not very efficient, is it? The fairly fat T-12 really doesn’t lend itself to good reflector design. In order to improve on this deficiency, the Power Compacts (PC) were invented. They are far narrower fluorescents than a T-12, so more bulbs can be squeezed into a reflector. The only problem is the tube is so weak that it must be folded into a U-shape to brace it, so much of the light produced crashes into the other side of the U, appreciably decreasing the light's output. That U-shape doesn’t lend itself to good reflector design, either; so PCs are a little better than T-12s, but not much. A couple of years ago a new diameter tube came into the aquarium market. It was the slim T-5, high output, with only a 5/8" diameter, but still a single tube like the T-12. Like a PC tube, this small diameter allowed more tubes to be packed under an aquarium hood, and its thin design allowed for much better reflectors to be developed. With a T-5 we can make a long reflector with a parabolic shape conforming to its diameter and, because the tube is so thin, use individual reflectors for each lamp. This translates to a more focused beam and, although still not as intense as a MH lamp, more light reaches the tank’s bottom.
Metal halides also have a couple of forms. Originally, the common type was the mogul base, with a tube that screws into a socket like a light bulb. This type of MH has a bulb within a bulb design, with the arc tube jacketed by an outer shield tube. This protects the arc tube while limiting the arc tube's UV emissions, which can be harmful at the low wavelengths it produces. Mogul base tubes also have two subsets: one form uses an additional starting electrode to start the arc plasma (probe start), and the other uses the arc electrodes themselves to start the arc (pulse start). The probe start claims to keep the high voltage jolts used to start the lamp from eroding the arc’s electrodes. The ballast employed must use the tube's starting system, or it will not fire.
The other major group of metal halides is the double-ended tubes, commonly called HQI (high quartz iodide). These are arc tubes with contacts at both ends, like fluorescents. The tube is fairly thin and has no outer shield tube. Please Note - This unshielded arc tube emits dangerous U.V. radiation and quartz glass does not block it. Do not look at the bare tube. Always have a plain glass or U.V. protected plastic panel between the arc and the viewer when using this type of MH. Untreated clear plastic, like the sides of an acrylic tank, is transparent to U.V. These are always pulse start as far as I know. HQI bulbs' main advantages are that more can be placed inside a hood, and their reflector design can be smaller, than mogul base lights. They do run hotter than mogul based MHs, and may produce slightly more light. I favor HQIs for most applications, but mogul-based tubes are usually cheaper.
One final word on using MH lamps to light your tank. When a MH lamp is started, there is an initial period when the lamp does not provide full output. This is because its plasma temperature needs to reach a certain level before the metal halides are fully vaporized. Depending on the lamp, that can take about 10-15 minutes. Another unusual thing is that if you turn the lamp off, you cannot immediately restart it. After shutdown, the hot lamp takes awhile to cool. While cooling, the metal halides remain in a gaseous state, and if a high voltage is applied the lamp will burn out. Metal halide ballasts have a timer circuit built in to prevent restarting for a set period. Again, this re-strike timeout period runs from 10-20 minutes.
Where does all of this leave us? Well, it means that in standard depth aquariums (under 30”), either system can be used. If your tank is deeper, then you should resign yourself to using only MH. Fluorescents just can’t handle that deep a tank. The choice is yours, but many reefkeepers tend to straddle the fence and use a combination of MH and fluorescent - the best of both worlds in many cases. In my opinion, if you can afford it, I recommend using a combination solution, rather than a single type of lighting.
Hang in there; we have just a little bit more to cover. If you look at the description of most any Tank of the Month’s lighting, you often see, “I use two 12K 175- watt MH supplemented by four T-5 actinic fluorescents.” What does that mean? First, let's discuss the “K” thing, which people often figure is Kilo, for thousand. Actually, we can thank Irishman William Kelvin for the K. Lord Kelvin has a temperature scale named after him. The units are the same as those in the Celsius scale, but zero starts at absolute zero. So they are talking about how hot the lights get, right? Sorry; it has nothing to do with the heat generated by the lights, but is a measure of the color of the light produced. Confused? Temperature of color? When talking about light we need a way to define the spectrum involved. Earlier I was using wavelength in nanometers to define a color, but the darned photographers wouldn’t know a nanometer if one hit them in the face (which is happening constantly), so they use temperature as a definition. A “red-hot poker” is hot, but not really hot. As we increase the temperature, we get yellow-hot, then white-hot, and finally, blue/white-hot. As the Kelvin temperature increases, the wavelength shortens and the color produced goes from red to blue and beyond. Scientifically, the K value is the apparent color of light emitted from a black body at a specific Kelvin temperature. Sunlight is what Clark Griswold likes when he's on vacation and, because the sun shines at around 5000-7000°K, he likes 555 nm for looking at things. In aquarium lighting we are interested mainly in lighting in the 6000-14,000K range, going from sunlight-white to a bluish white. This is the area where photosynthesis peaks. Both fluorescents and MH bulbs are available in K values of 20,000 and higher. Beware, however; not only do they appear dim to our eyes, which are not as sensitive to these high K values as they are to lower ones, but their true lumen output is usually much lower than their smaller K value counterparts. Now for a small nano tank, the choice is somewhat limited because we can mount only a single light over the tank. Here some compromise is needed, so bulbs with a higher K value need to be used in order to provide the blue-violet wavelengths required for photosynthesis. In those cases, 12,000-14,000°K bulbs are probably in order. Also, some PC tubes come with one side producing a fairly modest K white light and the other side generating a nearly actinic spectrum. I have been mentioning actinic in this thread, and it needs some explaining. Actinic itself means light that promotes a chemical reaction. In the 1980s fluorescent tubes started to appear that were just above the wavelength of “black lights.” They produced light in the area of 420-450 nm, and that happens to be a wavelength relished by photosynthetic algae. It quickly became apparent that these lights could be useful in aquariums, and especially marine tanks, where such a light would possibly enable aquarists to successfully maintain corals. That they did, and they were one of the innovations that have made reefkeeping possible.
Now in a large tank I like to mix and match. With a 100-gallon tank I may mount both MHs and fluorescents in the same canopy. This allows a choice of K values that cover a broader range than is possible with a single light. I like to use a couple of 8,000-10,000K MH lights for the center area where corals will be showcased. I then supplement those with actinic fluorescents. I use the actinic for the fluorescents because it gives them a "boost." Light in that wavelength penetrates water very well - far better than light in the middle to upper spectral range. The more powerful MH lamps are then used to provide the upper spectral emission, and their punch can drive that light deeper than fluorescents could. This is a balanced approach. Most reefkeepers burn all their lights about 8-10 hours per day, but set the actinics to come on an hour earlier and turn off an hour later than the MH, thus simulating dawn and dusk.
Even though MHs and fluorescents are today’s standards, we are perhaps seeing a new age dawn. Light Emitting Diodes (LED) have been around for some time; I even had an LED calculator way back in the early '70’s. Back then, colors were limited and output was abysmal. Not so today. Single diodes now can emit high spectral output whites, and their total light output climbs every few months. They emit very little heat and consume little electricity. LED hoods are now appearing that claim to be equal in light output to standard MHs and fluorescents. They are currently very expensive, but prices may fall as technology improves. Also, few usage reports have evaluated their performance in marine tanks. Do look for them to become more common in the next few years, though.
Other Lighting Considerations
Now that you have selected your lights, they need to be mounted over the tank. Many aquarists chose complete canopy systems, but our more industrious members custom-build their own. The two common methods for using lights are in a hood/canopy, or mounted to hang over the tank as pendants. Both methods have their virtues. In the pendant method the lamps are hung from the ceiling, in individual reflectors for MHs, or light strips for fluorescents. This method allows the lamps’ distance from the tank to be adjusted, and gives better ventilation than a canopy, which is an important consideration, especially for large tanks. The canopy method sets the lamps at pretty much a fixed height, and good ventilation of the canopy is a must.
As I just mentioned, heat is always an important factor when mounting a lighting system. Talk always arises about how much more heat a MH produces than a fluorescent but, in reality, their overall heat output is not that different for systems of similar wattage. The MHs produce all their heat in a small area, and the fluorescents spread their heat down their tube. What makes a MH lamp's heat affect the tank water's temperature more than does a fluorescent lamp's is that the former generates heat mainly as radiant energy. With a bank of fluorescent bulbs above the tank, most of the heat goes into the air in the canopy space, from where cooling fans can draw it away. MH lamps heat not only the air, but also transmit radiant heat energy through the air directly into the water column. In a small tank in a home with air conditioning, ventilation should keep the tank at the desired temperature. In large tanks, however, this may be a problem. It has to do with thermal mass. A small tank, say, a 2' x 2' x 2' cube, holds about 60 gallons of water. Counting the top, this cube's surface area is 20 square feet (I'm counting only five sides because the tank's bottom is insulated by the stand). That means we have 3 gallons of water for every square foot of exposed surface, and that exposed surface dissipates heat to the room. Now, if we increase the cube's size to 3’ per side, we have just over 200 gallons of water, with 45 square feet of exposed surface. That reduces the area from which to dissipate heat, because we now have 4.5 gallons for every square foot of surface area. This cuts our cooling rate by 50%. As the tank gets larger this drawback grows, and somewhere it will reach a point where another means of cooling the tank is needed. This means using a chiller, which adds greatly to the lighting's total cost.
One word here on mounting ballasts. Always choose remote ballasts when designing a lighting system. It is far better to have them in the stand than in a hood or canopy. They get less exposure there to humidity and spray and, if they are magnetic, they keep the heat that they generate away from the tank.
Well, this has gotten pretty long. I will close here with a final word on lighting. You have great lights and everything is fine, but a new coral you just introduced bleaches. Why? We are always preaching quarantine on Reef Central, but it is not only disease that kills. Exposure to excess light levels can also kill. Initially, global warming was blamed on coral bleaching on the world’s reefs. That still is probably true, but more and more researchers indicate that it is not only heat that kills. As the ocean warms, the corals seem less tolerant to intense light, and the symbiotic algae they host cease photosynthesis. The end result is that the algae are expelled and the coral dies. When a coral is added to a new tank, the water temperature may be far higher than when it was in transport, and it is certainly much brighter in a well illuminated tank. This can cause bleaching. The solution is to slowly acclimate the new coral to its new conditions. This can be started in the subdued light setting of a quarantine tank. When the coral is added to the main tank, you can raise the height of the lights above the water’s surface to reduce the lux and let the coral adapt. Something as simple as this can mean the difference between success and failure.
When aquarists understand how modern aquarium lighting benefits the maintenance of their reef aquarium, they can make wise choices that benefit their tank’s inhabitants and that please the eye of the beholder, without looting the beholder's wallet.
For more on this topic you can visit my old Lighting Thread, but be warned: it is even more rambling than this was, if you can believe that.
If you have any questions
or comments about this article, please visit this thread in the New to the Hobby forum on Reef Central.