A method is described that is particularly effective for the filtration of an aquarium by means of a filtration unit wherein a cartridge containing activated carbon is cyclically replaced by a cartridge containing material for the removal of nitrate and/or phosphate ions. A plurality of mechanical filters may also be alternately replaced, so that a colony of nitrifying bacteria is always preserved on one of the filters. Also described is a filtration unit that allows the application of the method and a replacement kit containing both the cartridge containing activated carbon and the cartridge containing material for the removal of nitrate and/or phosphate ions, and preferably also one of the mechanical filters.
