JBL Protemp E300 External HeaterIn line heater integrated into the hose of the external aquarium filter, removing the need for a heater INSIDE an Aquarium. Easy installation: cut through the hose from the external filter that feeds the water back into the aquarium, insert the external heater in between, insert the plug and set the water temperature you want. The integration in the filter circuit ensures a very even distribution of the water temperature with an accuracy of + 0. 5 C