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The most popular lighting for reef aquariums, or to be more specific SPS reef aquariums, continues to be metal halide. This is because metal halides emit a very intense light that has considerable seawater penetration strength as far as aquariums are concerned. Metal halide bulbs are also available in the Kelvin range desired by reef aquarists.

There are two major disadvantages to metal halide lighting systems, the first is cost as they are electricity hungry. The second is heat emission which could be a problem in itself or cause a minor problem to become a major one.

With air temperature warming up for the summer the heat emission from the metal halide bulb(s) doesn’t alter, but the increased air temperature does mean that the aquarium seawater will naturally warm up. This in itself isn’t a bad thing as the heaters will not activate as much saving the aquarist money. Unfortunately, heat output from the lighting can increase the seawater temperature even more, moving it passed the design level. The heaters will of course not activate but these high temperatures are detrimental to the aquarium, for a start it upsets the stability of the system. Increased seawater temperature also reduces the amount of oxygen the seawater can hold, so in say a heavily stocked fish only aquarium there could be trouble. If temperature climbed excessively trouble could occur in any aquarium system. The bacteria housed within the bio-filter that are dealing with the toxins in the seawater are also consuming oxygen which could exacerbate the situation.

So overall it is undesirable for the seawater temperature to climb excessively. There are ways of cooling things down though and with metal halide this is a need that is more likely to be encountered.

Some aquarists counter the effect of the metal halides by cooling down the seawater itself. They do this by employing a chiller (a cooler) which must be specifically designed for marine use because of the very corrosive action of seawater. The chiller needs to be sized correctly or it could be ineffective, a check needs to be made to ensure the device is able to deal with the gallons of seawater involved and also the temperature reduction required – as far as the latter is concerned, if the chiller is matched to the gallonage it should be adequate as there is a temperature adjustment range.

There’s nothing really wrong with using a chiller as described. However, if the sole use of a chiller is because of the heating effect of the lighting it is not really economical. The lighting is consuming electricity which cannot be avoided (except to ensure the bulbs are correctly sized and the lighting on period is not excessive). A chiller is quite an expensive piece of equipment and quite expensive to run. If the seawater heats up anyway quite apart from the lighting fair enough.

If it is mainly the lighting that is causing the seawater temperature problem it is better to tackle the problem at source. Some free hanging metal halide canopies have fans built in which help direct the heat away from the seawater. The biggest problem is likely to be found with those lighting systems that are enclosed within a hood which sits directly on top of the aquarium. These canopies are often an ‘all in one’ build with the canopies matching the aquarium stand.

If the canopy is as described it could have been supplied commercially or been a DIY project. Commercial ones sometimes have fans built in, but not always. Some have air vents at the top and that’s it. These air vents do release heat as warm air rises, but the heat release is too slow and doesn’t deal directly with heat into the seawater. Fans are reasonably easy to install.

There are two ways of installing the fans; the first is to have two fans, one at each end, with an air vent in the middle at the back, and the second is again to have two fans but no air intake at all. In the second version an air vent could be fitted either in the middle or above, it will not do any harm. The idea is to direct a cool air flow across the space between the metal halides and the seawater surface.

Metal halide canopies are of necessity quite large with a good space above the seawater surface, so there should be plenty of room to fit a fan at each end. Some aquarists use computer type fans but the danger with these is that they may not move enough air to deal with the heat. A better option is to use mains electricity powered fans. The ones that are useful are those smaller units that are fitted into bathrooms as extractors; they are usually a small plastic box with vents each side and the fan already mounted inside. Some of these type fans are reversible, that is the fan direction and therefore the air flow can be reversed. If two of these fans are obtained one can be fitted at each end of the lighting canopy. Two holes will obviously need to be cut but if the edges are a little ragged the fan units could fit on the outside to cover them up, the units need to be checked for this option. Some units have a removable outside cover which also fastens the main unit on the inside securely.

In the first option the fans are fitted and there is an air vent in the middle of the canopy back plate. The fans are set so that they both suck in cool air from outside the canopy. This causes an increase of air pressure – not a lot – inside the canopy and the air escapes through the air vent taking heat with it. Care has to be taken that the air doesn’t escape downwards towards the seawater or the cooling effect could be partially or wholly lost.

The second method, with or without an air vent, is probably the best. The fans are fitted at each end of the canopy but this time one fan, it doesn’t matter which end it is at, is set to suck air in. The opposite fan is set to blow air out. This means that there is a powered cooling air flow through the lighting canopy. If there is an air vent some air could escape, but again heat will go with it.

The paragraphs above are concerned mainly with metal halide lighting, but those aquarists who have fluorescent lighting such as T5’s or earlier types could be surprised at how much heat actually builds up in the canopy. A fan cooling system could be an advantage.

There is a major point that needs to be mentioned and this is concerned with safety. Electricity incorrectly used is dangerous. The fans need to be securely fitted and not subject to splashing. Wiring to the fans needs to be completed safely, with the wires going to correctly fused power outlets, and the wiring obviously needs to be properly secured, not trailing. Any doubt and advice from someone competent should be obtained.

Preventing the lighting system from overheating seawater is generally simple and can be done in a way which minimizes the additional electricity demand.


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