Abstract: An aquarium water heater includes a hollow heat-conducting internal tube with an electric resistance heater element and electrical connections disposed adjacent an outer surface of the hollow heat-conducting internal tube, a temperature thermal cut-out switch electrically connected to the heater element for overheat protection, and a heat-conducting, electrically insulating polymeric filler spaced between the internal tube and an external tube that circumscribes the internal tube and the heater element.