The Hydor THEO Heater line features PTC patented thermal technology which makes it among the safest aquarium heater lines on the market. UL listed by design, not by the use of accessories. Made from specially engineered shatterproof glass which, while not unbreakable, is designed to resist breakage in most normal aquarium situations. Highly accurate due to the use of precise micro-switch technology. Suitable for both fresh and saltwater.