LIGHTING FOR SALT WATER AND REEF AQUARIUMS

By Frank M Greco
Copyright 2000 Frank M. Greco, www.frankmgreco.com Reprinted with permission.
Aquarticles

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