Coldwater aquariums do not need a heat source. Generally the temperature maintained in most modern homes is adequate heating for these aquariums. However for warmer water dwelling tropical fish, that heat is not enough. Room temperature in most homes is usually several degrees below the optimal temperature for tropical fish.
Heaters/thermostats that are fully submersible are the popular choice for heating aquariums today. They are available in several sizes and can be found to match any size of aquarium. These heating units serve to heat up the water to a desired temperature then they shut off. The heater turns back on once the water in the aquarium drops to the level that has been set in the heater.