It all depends on the size and type of your aquarium and the type of fish you plan on purchasing. There are three basic types of heaters available. There is the aquarium heater that simply hangs on the side of the tank. The hanging types of heaters are the most basic heating unit and are okay for use in smaller aquariums.
There is also a submersible type of aquarium heater. This is the type of aquarium heater that is dropped into the water completely to heat and warm the aquarium. These are more convenient and more reliable that a heater that hangs on the side of the tank because it provides more consistent heat for the fish.buy aquarium heater manual