Wayne's Biological Sponge Filters are very dense and can harbor a very large amount of nitrifying bacteria.  The sponge measures 4" high x 4.5" diameter.  Unlike other sponge filters, these have an internal design that pulls the water from the base.  This provides for more suction from all areas of the sponge without the need for an air-stone.  In addition, the plastic is a softer material with snaps that prevent the pieces from coming apart after long term use.