Interpet Replacement Plain Foam is a form of biological and mechanical filtration. The plain white foam in your PF filter carries out two essential tasks to ensure the best quality water supply: Biological Filtration: in PF1 internal filters, the same foam acts as a media for critical bacteria to inhabit. This important bacteria breaks down toxic nitrate and ammonia caused by fish excrement and decaying algae and plants. Mechanical Filtration: The removal of waste such as uneaten food and plant matter in the aquarium. It is extremely important to clean the white plain foam only in water from the aquarium. Remove a bucket of water from your aquarium as part of a regular water changing exercise. Please rinse the foam until unblocked. It is recommended when replacing a new foam to cut the white foam in half to and place 1 new and 1 old piece of foam together to ensure a constant existence of bacteria. Each pack contains 3 foam filter pads