Abstract: An aquarium heater including a tubular plastic housing that houses a positive temperature coefficient element and two metal radiation fins and filled up with a thermal conductive filler, and a top cover capped on the tubular plastic housing to hold a power cable for transmitting electricity to the positive temperature coefficient element to generate heat. Because of the positive temperature coefficient element, no temperature control loop is necessary for controlling the temperature level. Further, the aquarium heater has a self-protection function because of the positive temperature coefficient element.