This is a sponge for the Air Panel Filters small (S) , wide (W) or Extension type.
Air Panel Filter is a new concept filter that can be installed at the back of the aquarium and works on the same concept of an underwater filter but the slim panel-like design makes it fit perfectly any aquarium. Filtration surface area is large that gives it an excellent filtration ability. In addition, it is now possible to replace the material easily by the pull-out filter material replacement function. By using additional units sold separately, filtration can be extended to bigger breeding setups.
Different media types can also be used as long as they can fit in those slim panels. These are the most famous air filters in Japan and breeders love them for breeding shrimps and fish.