I don't see any filter media at all in this photo. The blue bioballs are there to provide a surface for bacteria to accumulate. I use 100 micron filter material (almost like felt) and a more porous filter material on top of that. You can use a sponge as an initial filter, but put a 50 or 100 micron filter pad underneath that and change it weekly - keeps water very clear.
Roger (always pH) Lederer