Abstract: An aquarium heater including an elongated heater element including an insulating member and a wire heater extending from one end of the insulating member to the other end of the insulating member and with an outer sheath of insulating material having open and closed ends for enclosing the heater element at the closed end and having an inner dimension larger than the outer dimension of the heater element and with a pair of spacer members disposed individually at each end of the insulating member of the heater element for spacing the heater element from the inner surface of the outer sheath and with the spacer members including means for retaining the ends of the wire heater in fixed positions at each end of the insulating member and with a resilient sealing element disposed from the open end of the sheath to seal the heater element in the closed end of the sheath and with the sealing element including grooves for passing electrical wires for supplying electrical energy to the wire heater.