Rena Filstar XP 20ppi (pores per inch) foam is the first filter in a 5-step process used in the Filstar XP canister filter. Two foams should be placed in the bottom of the bottom basket as the water is filtered from the bottom up. This is an exact 8 pack replacement for the factory original filters included in the Rena Filstar XP Filters. These filters will wear out over time and the foam will loose elasticity and volume as the cell walls collapse. Replace these filters for continued smooth operation of the filter.