Although temperature was briefly touched upon in the discussion of water quality, a brief overview of the various types of heaters and methods of controlling temperature in the aquarium is essential. In considering temperature, always keep in mind that you will do well to re-create, as closely as possible, the natural temperature cycles that your pets would experience in the wild.

Tropical animals will, as a rule, be adapted to temperatures that vary little from season to season, while animals native to temperate climates are more tolerant of, and may even require, temperatures that vary throughout the year. Certain temperate species may experience enough in the way of temperature fluctuation if their tank is kept at normal room temperatures throughout the year (assuming that the aquarist lives in a temperate climate area). For others, hot and cool cycles may be regulated through the use of aquarium heaters.

The standard aquarium heater equipped with a thermostat, more properly termed a heaterstat, is familiar to most aquarists. Most of the models available today allow for precise temperature control. Of course, you must choose a heater that is appropriate for the size of your tank. Follow the manufacturer's suggestions regarding appropriate wattage. Remember that the actual amount of water that is in your aquarium will likely be less than the tank's capacity. For example, a 55-gallon aquarium will generally hold considerably less than 55 gallons of water once substrate, rocks, and coral are added.

Canister filters with heaters already incorporated within the unit are becoming very popular. Many, however, are designed for freshwater use only, because they contain metal parts. Marine animals are quickly poisoned when saltwater comes in contact with most metals, so be sure to check this point carefully before deciding upon a canister heater. A variety of more advanced systems are available for those with complicated or large aquarium systems.

Aquarium heaters do not give any warning of impending failure, so be sure to always include an extra one, set at the lowest acceptable temperature for your animals, within your aquariums. In the event of a failure of the main heater, the supplementary one will come on as the temperature drops. External heat sensors equipped with alarms and computerized systems are also now available for those who desire a safety net or a greater degree of control over the temperature of their aquariums.

As will be discussed concerning coldwater aquariums, chillers are now readily available to the aquarist. Be sure to purchase a chiller that is designed for aquarium use, because other types may bring the water into contact with potentially harmful metals or chemicals. Also be sure to check that the unit is appropriate for marine aquariums, because many models are designed for use with freshwater systems only.