Eheim phosphate out is a special iron compound for chemical filtration. The iron binds the phosphate removing it from the water circulation and thus reducing in stages the phosphate levels in the aquarium. In this way algae growth in the aquarium is inhibited. If the phosphate levels and cosequently the growth of algae starts to increase this means that the filter medium is depleated and must be replaced. Eheim phosphate out is availiable for external filters in graulate form and for internal filters as a foam filter cartridge. It is free from harmful substances and suitable for both fresh and saltwater.