So I'm looking for a cheap alternative to K1 and I hand some old, very course sponge laying around, I cut it up to about 1-1.5 cm cubes and it floats but when pushed below the water it takes a while for it to go back to the top. Has anyone tried to use this in a moving bed filter? I'll post some pics tomorrow but so far the only draw back I see it they may stick to each other a little. I'll see if I can't buy more and do a practice and post some results