I don't have one of those tanks but the dimensions would suggest using a 50, 75 or 100 watt heater. If the aquarium is kept in a warm room or in a warm climate then a 50 watt heater will be fine. If the tank is in a cold room then a 100 watt heater would be better.
All aquarium heaters need the heating coil and thermostat to be kept under water at all times. I could not see any space in the filter that would accommodate a heater. If you have a heater it would probably be best to keep it in the main aquarium.