It is very important to keep the aquarium properly heated at about 78°F. Get a reliable heater to avoid temperature swings, there should only be a 1 or 2 degree temperature change over 24 hours. The heater can be placed near an external power filter or other source of moving water to circulate the warmed water. To prevent hotter or colder areas in the tank the filter should take water from the bottom of the tank to the surface. Also, submersible ones may be placed horizontally for better performance.

Below is a chart of heater strength requirements based on tank capacity and desired increase over room temperature. Notice how the higher wattages recommend multiple heaters rather than a stronger one, ideally placed on opposite sides of the tank.