Ista's Hang-On Filter Inlet Sponges attach to the intake of hang-on back filters or even canister filters. This pre-filter sponge provides an area for beneficial bacteria to gather and acts as a first line of defense against waste and other contaminants. It also prevents baby fish and small invertebrates from being sucked into the filter. Invertebrates even enjoy grazing on the sponge itself! Easy to maintain, simply wash and reuse the sponge or replace as needed. Make the most of your filter by adding a pre-filter sponge to your filtration system.