The first shatterproof, shock-resistant glass tube heater on the market! This completely safe, submersible heater features the exclusive PTC (Positive Thermal Coefficient) heating film. There are no dangerous heating coils to deal with. May be mounted in any position under the water surface. Self-limiting heating system prevents the unit from overheating and damage should it accidentally run in dry air. Excellent for use in tanks with large fish and turtles that are likely to break most other heaters. The general rule of thumb is approximately 4 to 5 Watts per gallon of tank water. UL listed. 120 Volt, 60 Hertz operation.