Abstract: A thermostatically controlled safety heater for upright immersion in an aquarium includes a housing with an axially extending, heat-transmitting tube forming a chamber in which an electric heating element is disposed in radially spaced relation from the tube. The radial space between the tube and element, over substantially the axial length of the element, is filled with a heat conducting, thermally expansible liquid. An actuator device including a linearly movable plunger reactive to thermal expansion of the liquid is provided for opening and closing a switch in circuit with the heating element for maintaining the heater at a desired temperature. A hunting tendency of the switch contacts is overcome by providing a damping medium in the form of an air space having a volume less than about ten per cent of the volume of liquid between the plunger and the liquid. An expansible sealing diaphragm supports the plunger and isolates the liquid to the portion of the tube in which the heating element is disposed.