Aquarium Temperature
An important environmental variable is the temperature of the water in your aquarium. Fish are cold- blooded, and with very few exceptions their body temperature is nearly the same as that of the water they live in. Tropical fish are accustomed to live in water that rarely goes below 70°F. and so they usually get along at the ordinary house temperature of 72°F.
However, as the different species are accustomed to different temperatures, each fish must be considered individually. Seventy degrees is only a minimum. If the fish are expected to reproduce, some mechanical means must be used to raise the temperature of the water to the optimum temperature for breeding. To keep the temperature of the water as constant as possible at night, when the room temperature drops, and also to raise the temperature for the treatment of ich (white spot disease), a thermostatically controlled heater is used.
In North American pet shops, a single tube thermostatically controlled heater practically dominates the market. Some are completely submersible, but most hang from the edge of the aquarium. A few are built into the filter where the warm water is circulated by the pressure of the pump, but these units are rare. In England, Australia, and most of the rest of Europe and Asia a thermostat is available with outlets for several individual heaters. Each system has its advantages and drawbacks. A thermostat that controls many heaters may malfunction and destroy the fishes in several tanks. At least that's the main argument pet shop personnel give for not carrying separate heaters and thermostats.
