The requirements that must be satisfied by a filter and watercourse pump are quickly explained. It must be designed in such a manner that it can pump a particularly high volume of water, and the contaminants introduced into the pond through leaves, floating plants or animals, to the filter. However, one misunderstanding is that filter and watercourse pumps clean water. To clean water a filter is necessary that is adapted to the water volume of the pond. Selection of a sufficiently powerful combination is the key factor for the subsequent quality of water.