Hydor THEO (UL) Submersible Aquarium HeaterThe Hydor Theo&trade aquarium heater features PTC patented thermal technology which makes it among the safest aquarium heater lines on the market. The patented PTC heating system guarantees operation in complete security because if you run it dry there is no danger of damage. UL Listed by design, not by the use of accessories. The first shatter-proof heater thanks to the peculiar properties of our glass tube. This heater is made from specially engineered shatterproof glass which, while not unbreakable, is designed to resist breakage in most normal aquarium situations. Also shock resistant thanks to the PTC heating element and the shatter-proof glass tube. Highly accurate due to the use of precise micro-switch technology. Graduated scale, precise maintenance of set temperature. For use in freshwater and marine aquariums. This heater has free positioning! May be vertical or horizontal because it is completely submersible. Must be placed in water when the heater is operating. UL approved.