As we all know, the coral itself is not actually photosynthetic, but rather the zooxanthellae is. This unicellular symbiotic algae is what keeps the coral alive and beautiful. Without the proper light, these algae can be ejected from the coral (bleaching) or outright die, resulting in the total coral animal suffering, stopping of growth, diminishing or outright dying. To prevent this we must provide the correct light to the corals. Some corals can be kept with minimal lighting, but generally, high intensity lighting is required. The most important aspect of reef lighting is the photosynthetically active radiation, or PAR produced by the lights. PAR is the spectral range of sunlight used by plants during photosynthesis. The wavelength range is between 400-700nm with the most effective wavelength being around 420nm. Because PAR is so important, reef lighting must be able to not only produce this light, but also project it through the air and the water deep into our tanks. Another important consideration of reef lighting is the color temperature of the bulb. A reef bulb is rated by the color that it primarily produces and this color is based upon the kelvin temperature scale (°K). The kelvin scale is a physical property stating that as a black body is heated it will radiate different colors according to its temperature—in other words the hotter it gets, the different the color. Colors range from the pale yellows up to the violet blues. An example is a fluorescent bulb that is for a reception area would be a 3500k bulb while a classroom would be a 6500k. The 6500k bulb is whiter for better visibility. A bulb that would peak in the favorable PAR range would have a color temperature from 14,000k up.
Reef lighting is available in many different types, with each having advantages and disadvantages. Lighting is divided into three groups: 1) fluorescent, 2) metal halide, and 3) light emitting diode, or LED. Fluorescent lighting is broken down into two distinct groups; the straight tube style “T” lighting and the bent tube style power compact (PC). PC lighting is a high output fluorescent bulb but is being replaced by the more popular “T” style bulbs. The “T” style bulbs are just standard tubular fluorescent bulbs. These bulbs are sold as T12 or T5. The number refers to the diameter of the bulb in 1/8th of an inch, so a T12 bulb is 1.5 inches in diameter and a T5 is 5/8th of an inch. Fluorescent bulbs sold for reef aquariums are high output bulbs and therefore generate a large amount of light. T5 lighting seems to be more popular than T12, I assume due to size constraints. Fluorescents must run from a ballast and produce the most light when each bulb has its own reflector. Fluorescents are energy efficient, have a good life span, are very affordable, and come in a vast variety of colors temperatures. A downside to fluorescent bulbs is they do not produce “shimmer”. This is the effect of surface rippling being shadowed on throughout the tank.
Metal halides (MH) have been the go-to standard in reef lighting for many years. MH is a type of incandescent bulb where an internal filament is heated by electrical current and produces light. MH gives off intense light and is manufactured to produce given wavelengths. Metal halides are made in single and double-ended bulbs. These lights must be run from ballasts as well and each bulb must have its own reflector. MH’s produce some the best “shimmer”, but produce a lot of heat and use a lot of electricity. One danger of MH lighting is that they generate UV radiation, so shielding must be used in the form of quartz glass. MH lighting costs similar to high output fluorescent lighting.
The newest form of lighting in reef lighting is the light emitting diode, or LED. LED’s have been in use for quite some time, however, the intensity needed for reef lighting has just been developed in the past few years. Light emitting diodes are small solid-state electronic light bulbs that use semi-conductor alloys to generate light when an electrical current is passed through them. LED’s produce very intense light like a MH, but without the heat or energy consumption. LED’s are available in many color temperatures and they produce shimmer. The main disadvantage of LED’s is their cost. Costs seem to be about double of a comparable sized T5 fixture. Another area of concern is whether LED’s actually cause coral growth. Most users swear by LED’s and claim growth similar to MH but others claimed a lack of growth or even decline.