The Tetra Submersible Heater is safe (UL approved), compact, and easy to use. This heater requires no manual adjustment. Just plug it in, and the eletronic thermostat automatically adjusts and maintains your aquarium at 78°F. Since it s submersible, it can be easily concealed in your aquarium. It is also very durable since it is made of shatterproof glass. For aquariums up to 15 gal.