The aquarium hobbyist today is presented with an almost overwhelming number of options when it comes to lighting the aquarium. Fortunately, most manufacturers take great care in describing the qualities of their lamps. Bearing in mind the considerations that have already been discussed, such as photoperiod, wavelength, and light intensity, it should be relatively easy to locate a lamp that suits your particular needs.

Metal halide lamps are mainly used in saltwater aquariums, and their introduction into the hobby has revolutionized the keeping of marine animals, particularly light-sensitive hard corals. These lamps provide the high-intensity light necessary for many creatures that originate on coral reefs. The use of metal halides is particularly important in deep tanks where the lighting from even the strongest fluorescent tubes would be nearly filtered out before it reaches the bottom of the aquarium. Light entering the aquarium is lost in a number of ways, including through the glass, through refraction from the water's surface, through diffusion by particles in the water, and through a loss of intensity by absorption into the water, where it is converted into heat energy. Properly sized metal halide lamps largely eliminate these considerations.

An additional benefit of metal halide lamps, and other lamps of high intensity, is that the growth of potentially harmful dark green and red algae is discouraged. These types of algae thrive only in dim light. Once established, they spread rapidly, and they frequently smother coral and other sessile invertebrates.

The disadvantages of metal halide lamps are the relatively high cost and the huge amount of heat that they generate, but their price has begun to drop as more and more aquarists purchase them and additional manufacturers enter the field. Concerning heat, it is essential that strict safety precautions be in place. All flammable material should be kept far from these lamps, and children must be prevented from touching them. Metal halide lamps are generally suspended at least 12 inches above the aquarium so that the heat generated does not adversely affect the aquarium's water temperature. It is imperative that they be installed by a professional, because poorly anchored lamps can fall into the aquarium and cause an electrical hazard.

Be sure that your aquarium's lamp is installed in an aluminum aquarium reflector. Well-designed reflectors will deflect up to 50 percent of the lamp's light back into the aquarium.

There have been great advances in the development of fluorescent lamps for aquarium use in recent years. Several models are now available that provide the light wavelengths most suitable for plant growth in both freshwater and marine aquariums. One of the most useful lamps for marine aquariums is the triphosphor lamp. These emit light in the wavelengths required by most invertebrates and macroalgae, and they are calibrated to take into consideration the effect of the changes in intensity that light experiences as it travels through water.

Actinic blue fluorescent lamps produce the ultraviolet light that is required by many invertebrates and possibly by fish as well. They are also very useful for nighttime viewing of your aquarium's inhabitants. The presence of a blue light does not seem to disturb the animals as they go about their nightly activities. Actinic lamps may also be used to create a dawn and dusk period if they are set to go on before and turn off after the aquarium's other lamps.