The primary source of energy on the reef is the sun. Light drives the engine of photosynthesis and provides energy to corals by allowing photosynthetic zooxanthellae to survive in the coral's tissue. Zooxanthellae live within the coral tissues in a symbiosis - a relationship in which both the host and the symbiont mutually benefit. The zooxanthellae are provided a protected environment and use the coral wastes (carbon dioxide and nitrogen) as a food source. In return, zooxanthellae produce food for the coral in the form of amino acids and other organic compounds. Lighting also supplies energy to phytoplankton and other microorganisms at the bottom of the food chain. Replicating that energy source in our homes is a tough task.
There are three major technologies that have been shown to support our inhabitants, Metal Halide (MH), Fluorescent (VHO, PC, T5), and Light Emitting Diode (LED) Lighting Systems.
Metal Halide (MH, HQI) lights are by far the most intense type of lighting available second only to direct sunlight. They are the ideal point light source for our reef tanks, and have a very appealing shimmer effect on the tank. There are two major issues with Metal Halide. First is that the bulb replacement cost is high. Some bulbs cost over $100 and must be replaced yearly. Second, be sure to provide enough ventilation when using high-powered lighting because the bulbs run extremely hot, and can either overheat your aquarium or damage some part of the setup. Some fixtures are designed well and do not require additional ventilation, but those cases are exceptions to the rule.
There are two types of metal halide bulbs, single-ended and double-ended. The two lamps differ greatly in appearance in that the single-ended bulbs look like a large light bulb while double-ended bulbs look like a lighting filament the size of a chocolate bar. Single-ended bulbs screw into a mogul socket while the double-ended snap into a special connector. It is important to note that the double-ended bulbs do not have an outer UV shield and will require glass between the bulb and the viewer. They release above average amounts of ultra violet radiation that can be harmful to both the tank's inhabitants and people. The advantage that double ended bulbs have over the mogul socket bulbs is a more compact design, and possibly more intensity per watt. In our experience, a 250W double ended HQI was significantly brighter than a 250W mogul socket of the same wavelength when measured with a flux meter.
Fluorescent (T5, PC, VHO) bulbs have been a mainstay in the hobby for a very long time. While they are often used as an actinic supplementation to metal halide lights, they are more than capable of sustaining a reef habitat on their own. The benefit of fluorescent bulbs is the fact that they have a very beneficial spectrum that best brings out the colors of coral and fish. While metal halide bulbs tend to drown out colors with their intensity, fluorescent bulbs tend to highlight the colors. You do not get dramatic glitter lines however so many aquarists combine the two bulb types. Fluorescent bulbs tend to color shift faster then other types needing to be replaced every 6-8 months.
Light Emitting Diode (LED)
Solid state lighting is a relatively new addition to the lighting system landscape. The benefit of LEDs is their electrical efficiency. A 70W unit can put out as much light as a 250W metal halide. That reduction in electrical consumption can quickly lead to significant energy savings, especially if the LEDs are replacing several MH bulbs. The other benefit of LED systems is the longevity of the LEDs themselves. On paper, the diodes should last 30,000-40,000 hours which is essentially forever. The rest of the fixture will likely fail before the actual LEDs. Lastly, LEDs of different colors can be used for dramatic lighting effects. Newer models can be controlled to gradually change in intensity over the course of the day and some even come with red LEDs that can be useful for night viewing as many noctural tank inhabitants cannot see red light.
Unfortunately there are some downsides to LEDs. First, the cost of the units are relatively expensive. The cost of the LEDs themselves is inexpensive, but the rest of the fixture seems to be driving up the price. LEDs perform better when adequately cooled so many of the fixtures are basically huge fan-cooled heat sinks. LEDs have poor light spread, so specialized lenses are required to increase the angle of light distribution. Also, the jury is still out on LED's ability to actually grow coral. At the time this article was written, not many people have used LEDs long term. It is an exciting technology that will hopefully be both effective and more accessible in the future.
What Light Should I Buy? Long story short, it depends. What kind of lighting is right for you depends solely on what kind of critters you plan on keeping and what your aesthetic preferences are. If you are running a deep water reef with low light soft corals, and some non-photosynthetic gorgonians, it isn't a great idea to buy a 400W metal halide fixture to sit on top of your tank. The other extreme is trying to beat the odds on keeping a light demanding clam or stony coral under a couple of 40W fluorescent tubes.
When you figure out the kind of light appropriate for your inhabitants, try finding a tank with one of these types of light, and pick which one you think looks the best. All three work well and it will likely come down to aesthetics as the final determining factor.
Other Notes: Before you run out and buy the most intense lighting possible, check to see if your circuit can handle the additional amp draw. A four hundred watt ballast will run at roughly 4 amps, so you can imagine that running a few such bulbs along with all the other piece of equipment can be taxing on a standard 15 amp circuit.
Reflectors can be used to enhance the performance of most lighting systems. There are a variety of aluminum reflectors available for hobby use.