Saltwater aquarium hobbyists have a sometimes-overwhelming array of aquarium light bulbs at their disposal. However, each type of light has its own pros and cons, and the right bulb depends on the details of the aquarium you want to set up.
If you have only fish in your saltwater aquarium, you do not have to worry much about light. Saltwater fish do just fine with the same type of fluorescent bulbs found in freshwater aquarium setups. Regular fluorescent bulbs are sometimes referred to as a "NO" bulbs, short for normal output. You shouldn't need more than 1 1/2 watts to 2 watts per gallon in a fish-only or fish-only-with-live-rock saltwater aquarium.
Saltwater aquariums can have something no freshwater aquarium can: coral. Coral, and some other invertebrates, have symbiotic algae in their tissues, which help feed them. In order to keep these algae healthy, you need to provide them with strong light. Special types of fluorescents, like high-output (HO) and very-high-output (VHO) fluorescent bulbs fills this need. This category includes several variations. Power compact bulbs are similar to the compact fluorescent bulbs that are replacing regular light bulbs in many homes. Special T5 fluorescent tubes, have the same shape as regular fluorescent tubes but kick out more light. These lights require special fixtures and won't work in NO fixtures.
Metal halides work differently than fluorescents, but their powerful lighting puts them into the HO/VHO category alongside advanced types of fluorescents. They look like conventional incandescent light bulbs, but bigger. These kick off powerful light that works well in deeper reef aquariums. Since they produce a discrete point-source of light, their light ripples across the bottom of the tank like natural sunlight. However, these bulbs require a lot of electricity, and they heat up considerably. Some may need cooling fans to keep them from heating the aquarium water to dangerous temperatures.
LEDs are the newest trend in aquarium lighting. They use less energy than other types of lighting and produce less heat. Additionally, while other types of lights need to have their bulbs replaced yearly, LEDs can last for more than 10 years. The biggest drawback to LEDs is their cost. However, as more people adopt the technology and manufacturers increase production, the cost will drop. Ultimately, the savings on bulbs and electricity will make the expensive bulbs cost less in the long run.