There is no other practical lighting source for an aquarium that will provide as much light intensity in a small space as metal halide aquarium lighting.

This is why they've become popular for reef aquarists. The need for light intensity provided by metal halides becomes important as tanks become deeper. Generally, metal halides are practical for reef tanks 16 or more inches deep and become a necessity for tanks deeper than 36 inches. Even deeper tanks and more demanding stony corals, such as Pavona, Goniopora, and certain Acropora species need 250- or even 400-watt metal halides for success.

THE GLITTER EFFECT

Many people find metal halide aquarium lighting more attractive than fluorescent lighting types such as VHO or power compacts. Why is metal halide more attractive? In natural reef environments surface waves act as a lens that focuses light and creates "glitter lines." These glitter lines, familiar to divers, appear as flashes of light of high intensity and short duration and are very attractive to the eye. In reef aquariums these glitter lines or flashes can be created through the use of point source lights, such as metal halide lights, and surface agitation of the water. Fluorescent lighting is more diffuse and does not create these effects. Whether this flashing light is advantageous to the corals is not known.

HOW MUCH LIGHTING DO I NEED?

There is no easy answer to this. Experts recommend anything from a minimum of 1.5 Watts/gallon to a maximum of 6 Watts/gallon for reef tanks. The amount of light you need is very dependent on the types of invertebrates you have, the depth of your tank, and the spectrum of the lighting. Extremely deep tanks and tanks with a large number of stony corals might need from 4 Watts/gallon to 6 Watts/gallon. Tanks with just a few soft corals probably don't need metal halide lighting.

Regarding the corals, here are a few pointers:

• Small polyped stony corals, such as Acropora, definitely do better if metal halide lighting is used.
• Low light requiring animals, such as mushroom corals or non-photosynthetic gorgonians, or corals of the genus Tubastrea, will do well under regular output fluorescent bulbs.

A couple pointers regarding light penetration:

• The intensity of light decreases by the square of the distance that area is from the lighting source. Since metal halides must be at least 6 inches away from the surface of the water, and might need to be placed even higher above your tank to have the light reach the corals it is intended to benefit, halides have a disadvantage over VHO lighting that can be placed closer to the corals and spread more evenly using long fluorescent bulbs.
• The turbidity of the water can significantly reduce the actual amount of light that penetrates the surface.
• When activated carbon is used and is changed regularly, light penetration into the aquarium can be maximized.
• Cleaning or removing glass or acrylic lenses from the light hood will also help maximize light penetration.

METAL HALIDES AQUARIUM LIGHTING FOR SMALLER TANKS

Almost all metal halides are at least 175 Watts, and all the metal halides sold by marineandreef.com are 175 Watts and over, so they may be too much for smaller aquariums. However, if you use a Pendant, the fixture can be placed some distance from the water, thus reducing the intensity, and spreading the light out better over the entire aquarium.

OVERHEATING

Overheating is a potential threat when metal halides are used. However, with good air circulation provided by small fans and a cool air-conditioned house, increased water temperatures may not occur. Chillers can be used to offset any temperature gains. Whenever installing metal halides, be sure to make frequent water temperature checks.

SPECTRAL CHARACTERISTICS

For photosynthesis to occur, aquarium lighting should duplicate the spectrum of sunlight. The 5500K, 6500K and 10000K Metal Halide bulbs are full spectrum bulbs with all the colors of sunlight. Yet, the natural habitats of many popular reef aquarium organisms have a bluer light spectrum--a light spectrum around 7100K that is commonly referred to as actinic. (Deeper water is bluer because as one travels deeper into water the longer light waves, toward the red end of the spectrum, are filtered out.). For effective lighting you need both the full spectrum daylight provided by the metal halide bulbs along with the actinic lighting provided by the power compact bulbs.