Aquarium Components: Heater
A heater is an important component in maintaining tropical fish although they are rarely needed for more temperate or cool water species such as goldfish or koi. Each species of fish has an optimum temperature range, and a heater with a good thermostat helps maintain a constant water temperature in that range. Water temperatures significantly higher or lower than optimum will affect the fish's need for oxygen and food and its ability to survive.
Aquarium heaters are generally submersible, available in a wide range of wattages, and are relatively inexpensive and simple to operate. In a typically heated or cooled room, approximately 5 watts per gallon of aquarium water is generally sufficient for heating a tropical fish tank. A reliable heater and separate thermometer which are checked daily will help ensure fish are not stressed or killed because of a malfunction.