Fiberfill, or polyfil is like what is inside of a throw pillow on your couch. It's 100%polyester cottony fiber that you stuff inside of the media basket and it's so dense that it catches fish poo, food, floating sand, ANYTHING... It is a REALLY CHEAP water polisher. Since it is 100% polyester, there are no worries about chemicals or things that aren't necessary add ons like "phosban' or 'phosguard' or 'ammolock'... It's just like the bound filter media, or the mesh sponges, only finer so it actually collect microscopic debris and large debris too. I find amphipods loving in mine all the time when I change it. I place them in a cup of tank water, and dump them back in the tank after I change the filter. They seem to breed like crazy in the stuff!