A protein skimmer is a piece of saltwater aquarium equipment. The purpose is to remove proteins, ammino acids and organic matter from the water. It performs this task by injecting air into the water mixing chamber of the skimmer body. Proteins etc... are attracted by the air / water interface. The air bubbles travel up the mixing chamber and burst at the water's surface. The proteins etc... begin to build up and percolate up into the collection cup where they are taken away and disposed of.