How to Choose a Heater for an Aquarium. Heating a tank consistently is important for the health of your fish.
Be prepared for your heater to fail. One common problem is that the on/off switch gets stuck, leaving the heater on or off permanently. As a result, the water can get very hot or very cold, making the tank an intolerable environment for your fish.
Place two small heaters parallel to one another in the tank. That way if one fails, the consequences from heater failure won't be as disastrous, especially in the winter.
Choose between a traditional heater that hangs on the side or a submersible one. The submersible is the better choice since it can be put at the bottom of the tank, allowing the heat to rise and therefore be distributed more evenly.
Choose a heater of about 2-½ watts per gallon if the temperature in the room where the tank will be won't fluctuate more than 10 degrees either way. If it will fluctuate more than that, go with 5 watts per gallon.
Get a thermometer for your tank. The regular bulb thermometer either floats in the tank or hangs from the side and is sometimes hard to read. The other type is an LCD strip that adheres to the outside of the tank. Both types are somewhat unreliable and your best bet is to use both together.