Heating is essential in order to successfully keep a tropical aquarium, as room temperature is rarely tropical and normally far too cold to sustain a tropical environment for any duration of time. A heaters performance is measured in watts, to give an indication of the size of heater you will need for your aquarium match the size of your aquarium to the wattage of the heater.
A 100 litre aquarium requires a minimum of a 100 watt heater. Heaters are thermostatic so it is always recommended to get a slightly bigger one than required. For aquariums over 250litre it is recommended to install 2 heaters, as the workload will be shared increasing the reliability of the heating system. There are 2 types of heaters available, internal and external. Internals are positioned, when possible on a 45 degree angle, inside the aquarium. External heaters are used when an external canister filter is fitted and placed in line.