Filter media are an important part of aquatics. Filter wool is designed to absorb large particles of dirt. The different filter sponges clean the water from chemical pollution reduce nitrite or clean the water using live bacteria. JBL Uni Bloc contains 2 filter foam inserts for the mid-baskets. Both pads have a pore size of 20 ppi & are especially developed for bio-filtering. The round recess in the middle can be removed for special filter mass.