Hmm, I had planned ot put some in my ac 500, in the order prefilter, coarse AC sponge, filter floss, cell-pore but if it's no good I'll stick with sponges. I got some black sponge from big al's that looks pretty good. it's a lot finer than the coarse AC sponge. The "biomax" media that came with my new aquaclear seems really... not that porous. think it'd eventually grow some denitrification colonies?