Aquarists keeping cold water systems very often employ chillers (coolers) to prevent the seawater warming up excessively. Warm water systems that are in naturally warm areas may well need to do the same to prevent overheating.
Whatever the warm water system, be it a fish only aquarium or a reef aquarium, the aquarium temperature needs to be kept stable, with only a small deviation from the design point. The majority of aquarists will employ heaters, and the rest chillers, or maybe both.
Modern heater units and chillers are reasonably accurate in maintaining temperature, usually with a deviation of between 0.5 to 1.5 deg F. Seawater is fairly slow to cool and warm, so these changes are acceptable. Wider changes in temperature can cause stress to livestock, the worst affected probably being corals.
On the wild reef seawater temperature is for the most part stable throughout the year. Average temperatures on these reefs are 82 deg F *. Note that this is an average, meaning that some are cooler and some warmer. It gives an idea of where the temperature in an aquarium could be set. Setting the temperature in the aquarium to 82 deg F will give a general average temperature approximation. So is that the temperature to use? As said the temperature is an average – there are variances between reefs and there will be differences at various depths.
The lowest temperature that should be used is more easy to state. Coral reefs (of the warm water kind) do not develop if the seawater temperature is 65 deg F or less. So the temperature must be higher than 65 deg F. However, this is too low for organisms to have a high enough metabolism and it is most likely they came from much warmer water, so they would decline rapidly and die. The lower water temperature acceptable is 75 deg F.
Some aquarists set a temperature at between 80 and 84 deg F. This is done because the metabolism of the whole aquarium is raised – fish, corals, shrimps and the tiny life forms in and on deep sand beds (DSB) etc. This increase in metabolism should mean faster growth. Fish will demand more food, this food will be digested and waste will be processed more quickly. So all should remain more or less in balance. Well, yes, it should. However, the aquarist is moving closer to the edge. The aquarium has a small gallonage no matter how large it is when compared to the sea. The water can heat more quickly. If the temperature is already maintained above 80 deg F, then it is a quicker journey to higher temperatures and potential trouble. What if a heater malfunctions in the ‘on’ position? What if metal halides are in use (the heaters will turn off but the water could continue to heat)? Additionally, seawater that is cold has more oxygen than warm seawater. As the temperature continues to rise oxygen continues to reduce. This could cause trouble in any system, but particularly in a heavily stocked fish only one.
In a well managed aquarium the oxygen question should not arise. A protein skimmer is no doubt in use. Also, adequate water circulation will provide efficient gas exchange maintaining oxygen levels. The point is, the potential for a problem is there.
Apart from potential problems with the example oxygen, maintaining higher temperatures is going to cost more in electricity, except for the aquarist who has to keep temperatures down with a chiller. This is, or should be, a secondary consideration of course, it is the welfare of the livestock that is being considered (and the aquarist: if livestock are healthy and vibrant then the aquarist will be happy).
So, what is the best temperature setting? Lower down, the livestock will have a slower metabolism and oxygen will be more plentiful. Higher, the opposite. It seems a compromise is in order. I would suggest a temperature setting of between 77 and 80 deg F. This is somewhere in the middle of the acceptable range and gives a reasonable safety margin should a heater malfunction etc.
I run a soft coral reef at a temperature of 77 deg F. The corals grow at a reasonable rate – they need cutting fairly regularly. The fish display good appetites, and the DSB has a high population of miniature life forms. I should mention that I have never experimented with higher temperatures – but then, why would I when all is well.
Whatever the temperature, it needs to be stable. Constantly changing the temperature setting will do no good at all. If the aquarist wishes to experiment with higher settings, then the temperature should be increased slowly over a considerable period. The same applies to temperature reduction of any amount. The aquarist who sets a high temperature needs to ensure the heater/stats in use are of high quality, and preferably there is an external heater control with an in-tank probe in use.
(* Reference: Aquarium Corals. Eric H. Borneman)