I fill my bottom tray with Poly-Fil (pillow stuffing) since this is where the mechanical filtration occurs first. The other 3 trays I fill with ceramic bio-media.I have a blue sponge in the bottom tray, poly fil in the third tray, a white fine sponge in the second tray, and my bio media in the first tray. I've wanted to change it up for a while, just not sure what the best way to set it up.