Filstar XP 30ppi(pores per inch) foam is the second filter in a 5-step
process used in the Filstar XP canister filter. The 30ppi foam and will
trap finer particles than the 20ppi foam. These filters should be placed
on top of the 20 ppi foam. This is an exact replacement for the factory
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. Bulk packed 8 per box.