OASE Indoor Aquatics Pre-Filter Foam Set of 6 for The Biomaster 60 PpiThe pre filter foam set 60 ppi for the biomaster 250, 350, or 600 includes six ultra Fine replacement filter foams. The combination of coarse and fine filter foams allows you to customize to meet the filtration needs of your aquarium. It is best to replace the pre filter foam when you notice a decline in aquarium Filter performance. The Pre Filter Foam Set 60 Ppi For The Bio Master 250, 350, Or 600 Includes 6 Ultra Fine Replacement Filter Foams