Unlike most other topics in the hobby, lighting remains a very controversial and often misunderstood subject. This is because there are so many good options available to light a reef tank, and how a tank is lit depends not only on its size, but also what is going to be kept in it. It is now understood that a single strip fluorescent light is not adequate to keep many organisms long term, even if you are only keeping live rock and fish. However, properly lighting a tank does not need to be complicated. And when it is done properly, it not only provides a necessary energy source for the corals and other invertebrates, but also is aesthetically pleasing to the viewer.
Lighting in a reef tank needs to be stronger than in a tank maintaining only fish and dead corals because the water around the reef itself is very low in nutrients. The corals and other invertebrates have evolved to utilize the only energy source that is readily available: light. To do this, corals incorporate algae into their outer tissues to utilize the light present. During respiration, the coral gives off carbon dioxide and ammonia that is used by the algae during photosynthesis. The algae produce oxygen and carbohydrates as by-products. Energy is required for this critical process to produce nourishment for the reef; enough light must be present to provide that energy. That is why reef tanks require more light than fish tanks.
There are a wide variety of options available when it comes to lighting a reef tank. All of these options may make it difficult to make a choice. A reef tank should be lit as strongly as possible relative to the space available. The general goal is to have at least 3-5 watts of balanced light per gallon above a reef tank to provide adequate intensity. Choosing the type of lighting to use is a function of the tank’s size and the animals that are going to be kept. For large tanks or tanks housing stony corals, the best choice is to use high wattage metal halide lamps with or without blue fluorescent bulbs. For these types of tanks the wattage should be toward the 5+ watts per gallon range. In smaller tanks or tanks housing soft corals, there are many alternatives including moderate wattage metal halide lamps, as well as very high output (VHO), power compact (PC), or the new T-5 fluorescent lamps. Each of these choices has pros and cons to their use, but as long as the 3-5 watts per gallon rule is observed there will be adequate light for the corals to thrive.
In addition to intensity, lighting for a reef tank also needs to fall within the proper spectrum and be aesthetically pleasing. For these reasons, blue lights in the form of Actinic fluorescent tubes or high color temperature metal halide lamps are incorporated into the lighting systems. Actinic lamps are very blue in coloration and cause many corals to bio-fluoresce green when they are lit. When they are used alone the look they produce may be somewhat eerie, so they are usually used in a 1:1 ratio with daylight lamps. This combination produces a balanced lighting effect that is pleasant for the viewer and produces significant usable light for the corals. Recently, metal halide lamps that produce more light at the blue end of the spectrum have become available. These lamps do not cause corals to bio-fluoresce, but they do produce a blue cast to the aquarium. As a result they are usually used in combination with a less blue halide lamp when more than one lamp is used. These new lamps have color temperatures over 10,000 degrees Kelvin, with some capable of up to 20,000 degrees Kelvin. The inhabitants of a tank and their needs will vary, as will the aesthetic preferences of different owners, so the best way to determine which lighting choice is best is to view numerous tanks and copy a lighting choice that appeals to you, or just experiment on your own.
Lighting a reef tank properly usually is the first or second most expensive component of setting it up. When done properly, the health of the corals is assured and the tank is visually appealing. So a lot of time and research should go into selecting your lighting to make sure that it is just right.
Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers