This is the first sponge stage of the biological filter media. This sponge should be cleaned out when the white pads are replaced. They should be cleaned in aquarium water only ( NOT TAP WATER) as the chlorine will harm the good bacteria that has built up in the media) .They should only be replaced when unable to clean effectively. It should not be replaced at the same time as the other blue sponge ( Fine) as this would leave the aquarium with no effective biological filtration.
