The Tetratec Heating Capsule is the world's first external aquarium heater that inserts directly into the Tetratec Power Filter, creating an integrated heating and filtration system. Water is warmed as it is pumped through the heating chamber, preventing cold spots and potential accidents that can occur with traditional in-tank heaters. Made with shatter-resistant glass, exceptional electronic accuracy and automatic over-heat protection. Fits the TetraTec PF150 Power Filter.