The Oase Pre-Filter Foams for the Oase BioMaster External Aquarium Filter range are available in a range of densities to suit individual filtration requirements.30ppi = Coarse45ppi = Medium60ppi = FineSpecifically designed for the unique Oase BioMaster Pre-Filter system, the Pre-Filter foams effectively remove debris prior to water entering the main body of the External Filter, thus reducing filter maintenance.Oase BioMaster 250 External Aquarium Filter Contents:4 x BioMaster Pre-Filter Foam1 x BioMaster Filter Foam – Orange 30PPI2 x BioMaster Filter Foam – Blue 20PPI1 x Oase Hel-X 13 Biomedia 800mlOase BioMaster 350 External Aquarium Filter Contents:5 x BioMaster Pre-Filter Foam1 x BioMaster Filter Foam – Orange 30PPI3 x BioMaster Filter Foam – Blue 20PPI1 x Oase Hel-X 13 Biomedia 800mlOase BioMaster 600 External Aquarium Filter Contents:6 x BioMaster Pre-Filter Foam1 x BioMaster Filter Foam – Orange 30PPI3 x BioMaster Filter Foam – Blue 20PPI2 x Oase Hel-X 13 Biomedia 800ml