The protein skimmer is a combination of three different filter systems. The filter operates biological, mechanical and chemical. The principle of the skimming of salt water is not new and has been successfully used for years in aquarium filtration. Thanks to the unique, patented, design of the Tornado protein skimmer, it is now possible to apply the principle of protein skimming in fresh water. The surface of an air bubble attracts dissolved substances out of the water. Because of the adhesion of the bubbles these substances, like chemicals, waste particles, colourants and algae, are concentrated. By means of a venturi, air is mixed with water. By forcing this mixture through the filter material (the "KNS" bio rings) to a lower drain, there is a long contact between air and water. This changes the surface tension of the water continuously and creates a thick foam. This foam will be separated from the cleared water by a grid and exits through the foam outlet. Different kinds of pollution are caught in the foam and drained. This makes the filter self cleaning. Dissolved substances like; phosphates, ammonia, humus acids, colourants, proteins and a number of heavy metals, are filtered out of the water and drained together with the foam. The extraction of dissolved substances is not possible with normal filters and can only be done if chemicals are added.