Replacement foams for your PF Mini filter. The white foam in your PF filter performs two essential tasks that ensure optimum water quality, mechanical and biological filtration. This filter media will remove particulate waste and also promote beneficial bacteria. This beneficial bacteria can help to break down ammonia and nitrate caused by aquarium waste. It is advised that the foam is replaced every 4 weeks and that it is beneficial to cut the foam in half and use a mix of old and new filter media.