Powerheads are submersible aquarium pumps and can be used to create water movement or as a return pump for refugiums and other equipment. Moving water inside you tank is important to carry nutrients to plants and corals. It also eliminates dead spots that can cause detritus buildup from fish waste. Powerful new propeller style pumps such as the Vortech or Sicce Stream Pumps move large volumes of water and are great for reef aquarium flow.