For the vast majority of tropical fish, an aquarium water temperature of 76 to 78º Fahrenheit will be fine. Some species like cooler water and some prefer warmer water, but as a compromise, this range works well.
If you are having difficulty deciding what fish to buy, it wouldn't hurt to choose fish that prefer the same temperature range. Goldfish are not tropical fish and fare better at temperatures closer to 65º Fahrenheit.
It is very important that the water temperature be consistent. Rapid fluctuations in temperature, particularly down, cause physical stress to fish that often leads to disease. The solution to maintaining the correct water temperature is an aquarium heater and thermostat.
Aquarium heaters are available in a variety of types, sizes, and prices. When it comes to aquarium heaters, trying to save money is not a good idea. The reliability of a heater is too crucial to risk buying an inexpensive one.
The weak link in any heater is the thermostat, which regulates the heater, turning it on and off to maintain the desired temperature. The quality of design, materials, and construction of the thermostat is one of the things that separates unreliable heaters from good ones. In cheap heaters, the thermostat often sticks -- either open or closed -- and this can be disastrous.
When the thermostat is stuck in the closed position, the heater remains on, raising the water temperature. Unless you make a habit of checking the temperature each day, you may not notice there is a problem until the fish have died, either directly from the heat of the water or because the warm water is unable to hold enough dissolved oxygen to support them.
If the thermostat sticks in the open position, the heater never turns on and the water temperature begins to drop. How low the temperature will drop depends directly on the room temperature.
In the summer, when the room may be in the 70º to 80º Fahrenheit range, the temperature may not even drop at all.
Learn more about aquarium heaters on the next page.