Genuine Interpet PF4 Foam (3 Pack) Replacement Aquarium Internal Filter Media. These replacement foams are designed for the Interpet PF4 Aquarium Power Filter. Each box contains 3 foams. Mechanical filtration is the term used to describe the removal of particle waste from the aquarium, including uneaten food, dead plant matter, fish waste, and surfactants. Biological filtration refers to the essential bacteria that inhabit the foams and break down toxic ammonia and nitrite caused by fish waste, uneaten food, and decaying plant and algae matter. In your Interpet PF filter, the filter foam plays a crucial role. Replace these foams regularly to maintain optimal filter performance and clarity of your aquarium water.