Unlike warmblooded humans, fish are ectothermic, meaning they take on the temperature of the surrounding water. Since they can't maintain the body temperature at a chosen fixed level, the temperature must be maintained through a heater.
Heaters are only necessary if you keep tropical fish rather than marine fish. Coldwater fish like goldfish or koi prefer cooler temperatures (68º - 72º F) and don't need a heater.
Tropical fish are warm water fish, meaning they like to have their water between 75º F and 79º F. Heaters can help maintain this constant water temperature. Fluctuating water temperatures will cause stress on the fish, which will lead to a number of infections, and even death.
There are two types of aquarium heaters: A "hang-on" style heater and a submersible heater. A "hang-on" style heater provides basic heating of the water. Because the heater is not fully submersed in the water, it is less efficient than a fully submersible heater. Also because "hang-on" style heaters are generally less expensive, they have less precise controls and have fewer operating features.
A submersible heater actually attaches to the inside of the aquarium (typically with suction cups) and the entire heater is positioned under the water. Because the entire heating tube is under the water a submersible heater is more efficient and provides a more consistent temperature.
The rule of thumb when buying a heater is 5 watts for each gallon the aquarium holds. Typically, one heater is sufficient for an aquarium. However, for better heat distribution in larger aquariums, especially tanks more than 36 inches in length, it's best to have two heaters.
The heaters should be placed at opposite ends of the tank and each should be about half the total required wattage. For example, if you have a 55-gallon aquarium, you will have the best heat distribution is you put a 150-watt heater at each end of your aquarium. If you have a 10-gallon aquarium, one 50-watt heater placed in the center of the aquarium will be enough.
If you use a power filter or a canister filter with only one heater, place the heater at the opposite end of the tank from the filter. This will help ensure good distribution of the heated water.
Aquarium heaters come in different tube lengths to accommodate different height tanks. Because heat rises it is important that you pick the right length heater so that all the water is evenly heated.
While most aquarium heaters can be set to the desired temperature, is always a good idea to buy an aquarium thermometer. The thermometer will help you make sure your heater is functioning properly and maintaining the temperature that is best for your fish.