The Fluval Pre-Filter Sponge prevents small fish & debris being accidentally sucked into the filter. Small creatures such as delicate bee shrimp can easily get trapped in the filter & can´t find a way out. An additional advantage of this sponge is the thin biologically active film which forms its surface of the sponge & provides a grazing area for shrimps. Follow this link for further information on Fluval Edge Aquariums Fluval Edge Nano Aquarium