Heaters are a necessary factor for keeping tropical fish. Most tropical fish live between the temperatures of 75°F and 80°F. Heaters perform as you would expect, heating the water safely so that tropical fish (which are cold blooded) can live in comfort. They generally consist of a glass tube housing a heating element, with controlling circuitry packaged alongside. Heaters are divided into two main types: non-submersible, and submersible. 

Non-submersible heaters are inexpensive, and clamp to the rim of the aquarium with the heating element in the water inside the aquarium. They are controlled by a bimetallic strip that bends depending on the temperature. If bent far enough, the strip touches a contact, which completes the circuit to turn the heater on. These heaters are adjusted by a knob on the top of the unit that sets the starting position of the bimetallic strip.

Submersible heaters operate in a similar fashion, but generally have more reliability and features. These heaters can be completely submerged in the water, because they are well sealed to prevent water from entering. In salt water, submersible heaters are the only choice due to the corrosive nature of the salt. Non-submersible heaters would fail quickly under these conditions. Some of these heaters have solid state circuitry which ensures better reliability and accuracy for temperature.

Submersible heaters tend to cost more than non-submersibles, but the length of reliable operation more than makes up for the cost. Some of the newest heaters employ titanium alloy construction for the tube, virtually eliminating breakage problems.

When determining heating options for an aquarium, the rule of thumb to follow is 5 watts per gallon of water. Therefore, a 20 gallon tank would require a 100 watt heater. As the tank gets larger (i.e. in excess of 100 gallons), this ratio can be reduced, and is actually a better idea to utilize more than one heater instead of one large one, as this will ensure even temperatures across the aquarium.