High quality sheets of reticulated foam can be cut to any size giving a large economical filter surface. Sponge set comprises 1 x coarse, 1 x medium and 1 x fine, to filter out different sized dirt particles. Foam should be inspected regularly and replaced gradually to maintain healthy populations of good bacteria. Fitting instructions: The black coarse foam is positioned at the top, the medium blue foam is positioned in the middle and the green fine foam is positioned at the bottom. This will allow for maximum particle removal.