From The Aquarium Wiki
Temperature is a physical property that is often described as how "hot" or "cold" something is.
It is commonly measured in one of two systems; Celsius and Fahrenheit. Celsius is used across the world, while Fahrenheit is used only in the U.S. The freezing temperature of water is 0°C or 32°F. The boiling point of water is 100°C or 212°F. To convert between the two systems, there are two equations. To convert from Fahrenheit to Celsius, use the equation . To convert from Celsius to Fahrenheit, use the equation Thermometers are used to measure temperature. There are several kinds of thermometers used with aquariums, including glass alcohol, adhesive plastic strip, and LCD powered thermometers. These should be placed somewhere one can see them; otherwise they are useless.
Relation to fish
Temperature is key for the optimal condition of fish. Since fish don't genetically originate from your aquarium, they are naturally adapted to their original environment. Temperature is one of the factors replicated that are replicated from a fish's environment. Fish can die because of mismatched temperature because they are ectothermic, or cold-blooded. Maintaining the aquarium's temperature involves the use of a thermostat and either a water cooler or heater.