First off,They dont take all the waste out of the water.Just excess nutrients,some fish waste,and dissalved protiens.They work by mixing millions of micro bubbles with tank water in a cylinder.The waste then sticks to the outseid of these bubbles and float to the top of the cylinder.When enough waste is collected on the bubble,it will not pop when it reaches the surface.Then these unpoped bubbles start to stach up,pushing them into a collection cup at the top of the skimmer.