Metal halide lighting is very popular with reef keepers, particularly those who keep hard (SPS) corals. This is because it is a very powerful light that can penetrate deep into the aquarium provided that the bulb wattage has been correctly selected. Metal halide is likely to be superceded before too long by LED arrays, but at the moment metal halide is the most popular for the reef.
Even though metal halide is so good at lighting reefs there are disadvantages. There are two major ones, the first being that metal halides are electricity hungry and will add significantly to the aquarium energy cost. The second is that they generate a lot of heat. It is this second disadvantage that is being considered.
The lighting has to be hung well above the seawater level and this is because should the protective glass or bulb be splashed it could crack as it is so hot. It needs to be remembered that if the glass is to be wiped over with a damp cloth it needs to cool down first.
We nearly all like sitting in a favourite spot in the sun, feeling the warmth. Metal halides bulbs are a little like the sun in that they radiate considerable heat and unfortunately this could cause trouble. Many reef keepers use cooling apparatus to keep the temperature of the seawater under control though this for most is only in summer. This apparatus could be fans in the lighting hood, a pedestal fan alongside the aquarium, a ‘chiller’ (electric seawater cooler), or perhaps a combination. Stability of seawater parameters is important and this includes temperature. If the temperature climbs too high livestock welfare is in jeopardy. If the temperature is noted to be climbing much too high then the first action is to turn off the metal halides until cooling equipment can be used. The aquarium shouldn’t be plunged suddenly into total gloom, so the blue (actinic) fluorescents can be left on.
There is another danger from metal halide heat as well, and I wasn’t aware of this until recently. There are aquarists who use acrylic aquariums and these aquariums, like glass ones, have stress bars built in across the top. It is reported that the major failure with acrylic aquariums is where a metal halide bulb is directly above a stress bar. The heat softens the acrylic which then stretches under the outward pressure of the seawater and splits or worse could occur.
So if it is possible to move the bulb away from the stress bar this should be done. If the metal halide is then off centre this could be all right, check the light coverage. The guideline is that a metal halide bulb at the correct height above the aquarium should light three feet of aquarium length.