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Frank M. Greco
Title: Lighting for Salt Water and Reef Aquariums.
Summary: Frequently asked questions about lighting. A detailed article about the different equipment and how to use it.

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Date first published: 2000
Publication: Frank's Aquarium -
Reprinted from Aquarticles:
February 2003: Posted on Jesse Hunt's web site, Aquarium Information Source 
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Frank M. Greco,
c/o The New York Aquarium,
Boardwalk & West 8th Street,
N.Y. 11224
#205 - 5525 West Boulevard
Vancouver, British Columbia
V6M 3W6


By Frank M Greco
Copyright 2000 Frank M. Greco, Reprinted with permission.

Lighting for aquariums can be a very confusing topic. There are many opinions out there; some based upon hard data, others upon personal experience. Which lighting to choose: NOrmal fluorescent (NO), Very High Output (VHO) fluorescent, metal halide (M/H), compact fluorescent (CF) or a combination thereof? How many fixtures? What wattage bulb(s)? All of these questions must be answered before you go out and spend a lot of money on a lighting system that may or may not be adequate for your tanks’ inhabitants.

By asking yourself the following questions, you will, in the long run, save lots of time and money.

1. What type(s) of organisms do I want to keep?

This is the prime factor to consider when purchasing as lighting system. If you are going to try your hand at a reef tank containing primarily mushroom anemones and soft corals, VHO lighting, even in a deeper tank, may be sufficient (although CF or MH lighting would be even better). If you are going to keep predominantly stony corals, especially SPS (small polyped stonies) corals like Acropora, Stylophora, and the like, compact fluorescent or metal halide lighting are perhaps your best options, especially if you wish to keep the intense color seen in many of these species. There is one last option, that being that you will want to light the tank simply to view the inhabitants, in which case NOrmal fluorescent lighting will be sufficient.

2. How long and wide is my tank?

If you are using fluorescent lighting, the number of bulbs needed becomes more complex as the limiting factor becomes with width of the tank. In order to achieve the intensity required by some stony corals, you may need three or more VHO fluorescent bulbs (NOrmal fluorescent bulbs are not advised except on extremely shallow aquaria or tanks where no hermatypic corals are to be kept). Compact fluorescent lighting comes pre-set for various tank widths and lengths, and there is little guess-work when using this type of fluorescent lighting. Whenever possible, compact fluorescents should be utilized in place of VHO lighting. When dealing with metal halide lighting, the number of lights you will need depends, in part, on the length of your tank. In general, it is safe to say that if your tank is 2’ long or less, one metal halide fixture should do. However, if your tank is longer (3’ or better) you should have at least one metal halide fixture per two foot length (i.e. for a six foot long tank, three metal halide fixtures should be sufficient). If the tank is overly wide (> 2’), you will need to take that into account as well, and add an additional lighting fixture or two to compensate for the added width.

3. How deep is my tank?

This is THE most important to know when deciding on a lighting source. Forget about using the old adage of “watts per gallon”, since with today’s variety of lighting sources it has, in my opinion, little validity. My personal rule of thumb is this: that if the tank has a depth <16”, you might consider NOrmal or VHO fluorescents, depending on what it is you want to keep. If the tank depth is 16” to 20”, VHO or compact fluorescents should be used to achieve light penetration to the bottom of the tank. Anything greater than 20”, use either compact fluorescent or metal halide lighting, depending upon the depth. A good guide to choosing the proper wattage is that for tanks 16” or less in depth, anything from a 110 watts to 220 watts will be sufficient. For deeper tanks, use either 250 watt or 400 watt bulbs.

From research I have done, it has been noted that NO and VHO fluorescent lighting do not penetrate as far as and with the same intensity of equal wattage of compact fluorescent and metal halide lighting. In fact, it was noted that half the intensity of NO fluorescent lighting was lost within the first 6” of depth while with VHO fluorescent the same loss occurred within the first 12” of depth. Power compact fluorescent lighting lost half its intensity at approximately 19” of depth. With metal halide lighting of equal wattage, this same loss did not occur until 23” of depth. This is mainly due to the fact that any fluorescent light source is considered to be as line light source, and produces a diffused light. Metal halide, on the other hand, is considered a point source light. Water is a diffusing medium, so by using an already diffuse light source over a diffusing medium, you are further diffusing the light (i.e. making it less intense). This situation, while it does occur with metal halide, it not as obvious as a point source light has greater “punching power”. This is not to say that metal halide lighting has no place in tanks <16” deep. Quite the contrary. It has been noted that using low wattage metal halide, even in shallow tanks, increases coral growth and promotes color intensity, results that are not always seen with other lighting sources.

4. What type of housing should I use?

There are several varieties of bulb housings available to the hobbyist, with each style providing a different area of coverage and reflection rate. Each fixture throws off its own “cone of light”, and this needs to be taken into consideration when choosing fixtures. Also, each shape has its own reflective properties that are a consideration as well. When using metal halide, you should purchase fixtures that are parabolic in shape if possible. This type of fixture will direct most of the light down into the tank. Pendant metal halide lighting is a good example of this. The disadvantage to this is that light is focused into a somewhat narrow area. There are wider curved fixtures that throw a somewhat focused beam of light into the tank, and these work out well for the majority of uses. Boxlike fixtures will also work providing you can replace the (usually) flat reflective surface with a curved one. With NO or VHO fluorescent lighting, you have little choice in the fixture style. The shapes most commonly available appear to be adequate to the task at hand. Compact fluorescent lighting, on the other hand, provides fixtures which, while not as good as parabolic reflectors, provide more focusing of light than do other reflectors used with fluorescent lighting. In order to get maximum reflection into your tank, make sure the reflective material used in fluorescent fixtures is made of polished metal or an equivalent material. Plastic reflectors tend to yellow over time, and reflect less and less light as they age.

5. What type(s) of bulbs should I use?

When choosing any type of lighting, you should use a bulb whose Kelvin rating is no less than 6500 K. Lower Kelvin ratings will provide you with a light that is yellow to very yellow, and will throw off the color rendering a bit. With reef tanks, you will find that you may need to supplement this lighting with actinic 03 bulbs to correct the spectrum. It is my opinion is that bulbs of 6500 K to 10,000 K are adequate for most situations, although some hobbyists, including myself, have used 5400K bulbs with great success. There are 20,000 K bulbs available, but they tend to be quite blue and, as in the case of a <6500 K bulb, the color rendering may be thrown off. If you decide to go with a 20,000 K bulb, you may find that you will need to supplement with one or two NO or VHO full spectrum bulbs to help improve the color rendering. 20000K bulbs have much less red in them than do the lower Kelvin bulbs and tend to make things look very blue. When choosing a compact fluorescent system, you should attempt to find bulbs of a color temperature of 6700K and 7100K (if you like info on this, I can post that as well). A 1:1 combination of these bulbs is ideal for reef tanks.

5.1. Are actinic bulbs necessary? (reef systems only)

The next question that comes up is if supplementation of actinic 03 radiation (that is, lighting that ranges from ~380 nm to ~480 nm, with a major peak at ~420 nm) is needed with the higher Kelvin bulbs. In my opinion, if you are using a bulb 10,000 K or greater, supplementation is not absolutely needed. HOWEVER, the increase in coral growth and color provided by this supplementation makes it well worth the additional expense. Supplementation is advisable if you are using bulbs of 6500 K or less, and especially if you are using lighting of 5500 K or less. When using fluorescent lighting, you will need to use a combination of bulbs in order to get both as proper spectrum for the corals and a good color rendering for you. Usually a 1:1 ratio of actinic to full spectrum should be sufficient for most purposes. If you decide to use compact fluorescent lighting, a 1:1 ratio of 7100K and 6700K bulbs is, in my experience, your best choice for reef systems as this combination closely resembles that of the absorbence spectra for zooxanthellae.

In clear reef environments, the wavelengths of maximum penetration fall between 440 nm and 490 nm. This may explain, in part, the spectral peaks of chlorophyll b and some carotenoid pigments. This is not to say that other wavelengths are not represented or utilized by zooxanthellae, just that their energy relative to the above-mentioned range is not as great, especially as one goes deeper. It must be remembered that the greatest absorbence occurs within the relatively narrow range of ~450 nm to  ~460 nm. So, where does this leave the reef hobbyist? While the actinic 03 bulb is adequate to the task, the 7100K compact fluorescent tubes appear to be superior since it peaks at ~460 nm (with a smaller peak at ~420 nm), matching more closely the absorbence spectra of zooxanthellae. Utilized with a compact fluorescent of 6700K, the spectrum matches very closely that of the absorbence spectra for zooxanthellae, with peaks in both the ~400 nm to ~550 nm range and a smaller peak in the ~650 nm to ~700 nm range. In theory, then, the zooxanthellae are able to utilize a greater portion of the light hitting them than they might using an actinic 03/full spectrum bulb combination. Practical experience has shown me, as well as a host of other hobbyists, that the use of these bulbs in combination allows for superb stony and soft coral growth and color, matching those produced by metal halide lighting of similar wattage.

6. Should I use a reflector with my lights?

The answer to this is a resounding yes, no matter what type of tank you are keeping. Fortunately, most fixtures come with a reflector already built in. You want to use a reflector that is made of polished metal, as this will provide the highest amount of light reflection. Many reflectors are painted white which, while it does reflect light (not as much as polished metal, though), tends to diffuse the light as well and so is not as desirable as a reflective material. If at all possible, you should place the reflector so that it is parabolic in shape, as this will provide maximum reflection. Next best is a simple curve. The least effective is a flat surface although it will do if no other option is available.

7. What about fluorescent bulbs with the built in reflectors, or “twist” type bulbs. Are they really that much more effective?

While both are indeed more effective, they are not that much so and may not be worth the additional cost. In the case of a “power twist” type bulb, you may get an increase in intensity of 10%. This means that with a 40 watt bulb, you will get an output equivalent to that of a 44 watt bulb. Bulbs with built in reflectors also provide additional intensity, but perhaps not as much as is thought. While light is indeed “lost” to the tank from the top of the bulb (unless a reflector is used), it must be realized that this light, bouncing off the reflective material, is then passed AGAIN through the bulb. By then, there is a slight loss of intensity. So, the desirability of such lights in really up to the individual hobbyist, but in my opinion a good in-fixture reflector is far superior to a bulb with a built in reflector.

8. How high should my lights be above the tank?

This really depends on the type of light you are using. With fluorescent lighting of any type, you will want the fixture as close to the tank as possible so that way the maximum amount of light will reach the organisms. With metal halide lighting, generally speaking, 6” above the water is sufficient. Remember that for every foot above the tank, you will lose half the intensity. The closer, the better. HOWEVER...if your corals are not use to the more intense metal halide light (going from VHO to metal halide, for instance) or if you are changing bulbs, going from a lower Kelvin rated bulb to a higher one or just changing the bulb after 2500 hours of burn time, you may want to bring the lights up a bit higher, perhaps 12” to 18” off the tank, to give the organisms time to adjust to the increased UV output. Then over a period of a few weeks (how long will depend on how your corals react to the new lighting), lower the lights until they are once again 6” above the tank.

9. What about using UV shields on metal halide lamps?

Is this a necessity? In my opinion, no it is not. Most shields will block out a good portion, if not all, of the UV and possible the actinic 03 radiation put out by the bulb. In some corals, especially the more colorful species, it is the UV that provides the need for UV protective pigment production. It is these protective pigments that provide the intense colors we see in our corals. Without the UV, I find that the colors tend to fade with time. If you decide to go sans shield, you MUST protect the bulb against direct water contact. A slight bit of water spray, such as from an airstone, will usually not bother the bulb, but a direct splash may indeed shatter it.

10. How long should the photoperiod be?

Under normal circumstances, a photoperiod of 8 to 10 hours should be sufficient. More, and you run the risk of an algae bloom and/or stressing your corals due to an extended photoperiod. With less, your corals and freshwater plants may not do as well due to a lack of sufficient photoperiod. The easiest way to regulate photoperiod is to place your lights on a timer. This allows for an automatic day/night cycle without you needing to be there. Some reef hobbyists have arranged their lighting so that the actinic 03 bulbs come on first, then the metal halide(s), this being done so as  not to “photoshock” the corals. It should be remembered that although blue light may not appear bright to us, it IS bright to the corals so there may be no benefit to doing this. One problem that faces hobbyists is that they are not home a good portion of the day. To compensate for this, they leave the lights on well past the 10 to 12 hours actually needed. Here again, the timers come into play. It may be to your advantage to set the timers so that the lights come on at 10:00 a.m. or 11:00 a.m., and go off anywhere between 8 pm and 11 pm.This way, the organisms in question get the light they need, and the lights are on when you are home to enjoy the tank.

11. How often should I change my bulbs?

Generally speaking, ANY bulb should be changed after 2500 hours of burn time (3000 hours at most). Much after this, the intensity decreases dramatically, and you tend to have a spectral loss at the blue/UV end. Changing bulbs at a regular interval will avoid this. As an example, suppose you are burning your lights 10 hours a day. Assuming 30 days per month, you would want to change the bulbs every 8.5 months at minimum or 10 months at maximum. Do NOT assume that just because a light is bright means  that it is still valuable as a light source. This is not always the case. Our eyes are far more sensitive to the red/yellow/orange areas of the spectrum, whereas the greatest loss occurs down at the blue/UV portion (a portion of the spectrum to which ours eyes are not as sensitive. That’s the reason high Kelvin bulbs look dimmer to us).

Over time, as the bulbs dim, your corals become accustomed to the lower spectral intensity. Suddenly, as you change the bulbs, these animals are blasted with higher levels of UV radiation as well as visible light. This is the primary cause of “coral burn” or coral bleaching in captive specimens. In order to avoid this, whenever you install new lighting, or when changing from a lower intensity bulb to a higher intensity bulb, you might want to raise the fixture higher over the tank. Then over a period of a week or two, slowly lower the fixture back to its original height this applies mostly to metal halide lighting as other lighting sources do not seem to produce the same effect). This will give the corals time to adjust to the greater intensity. If raising the lighting is not an option, placing a sheet of glass or UV blocking acrylic between the bulbs and the corals should suffice.