From Shirlie Sharpe,
Your Guide to Freshwater Aquariums.
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A. In answer to the reader question:
"It is hot in England at the moment. My tank temp is 27.3 C (81 F). I keep guppies, mollies, angelfish, silver sharks, and a clown loach. Can anyone tell me what I can do if the water in my tank gets too hot?"

Heat can become a problem if it is markedly elevated for a considerable period of time (which might be possible during a prolonged hot spell). The Angelfish, Guppies, Mollies, and Silver Shark, will have absolutely no problems with the current temp of 27.3 C. The Clown Loach enjoys water as warm as 30 degrees C (86 F), and is probably thanking the weather person for the lovely warm water.

As long as the water temperature does not remain above 30 C (86 F) for weeks on end, you need not be concerned. However, as the temperature rises, the oxygen saturation in the water tends to fall. If you have a good filtration system, odds are you don't have a problem. Never the less, it won't hurt to increase the aeration to assure proper levels of oxygen. I'd also perform water changes more often, using water that is a degree or two cooler than the tank water. That will serve to keep the water temperature down, and maintain adequate oxygen levels for your fish.

In the event of a lengthy heat wave, you might have to take steps to cool the water. Here are some suggestions:

Keep the aquarium lights turned off.

Make sure the room does not receive direct sunlight.

Remove the hood and lid from the tank (use caution if your fish are jumpers)

Place a fan so that it blows directly across the water.

Float icepacks in the water.

Considerable debate exists over the use of a small icepack inside a box, power, or canister filter. Certainly an icepack will cool the water, however the drastic temperature difference traumatizes the bacterial colonies growing on the filter media. Personally I would not recommend ice or cold water packs inside a filter.

An alternative for aquariums with a canister filter is to lengthen the outlet tube enough so that it may be placed in an bucket full of ice. The water will be chilled after it leaves the pump, which will not affect the bacteria on the filter media.

Regardless of which method you use, be sure to keep the rate of reduction slow - about one degree Celsius (2 degrees F) every eight to ten hours. Remember, rapid water changes are harmful to your fish.

Good luck with your fish, I hope it cools off soon. Drop by the forum and let us know how things are progressing.