Submersible aquarium heaters with plastic casing are shatterproof and protect the internal components against thermal and mechanical shock. They are extremely strong and durable. If the shape of the heater is cylindrical then heat is evenly distributed throughout the heater. Durable glass constructed submersible heaters have a convenient temperature dial on top, which makes it very easy to read the temperature setting as the window is visible through the glass on the site. Submersible Aquarium Heaters ranging from 25 to 250 watts are ideal for fresh water and salt water aquariums.