just bought the newest filter from fluval at a promo price of $350. it's the FX5 and it is suited for up to 400 gal. it's on my 125 and it's the balls. anyways, I have 3 trays that I can fill with anything. I chose to use biomax spheres from my ac110s for 2 of the layers and this cotton stuff for the first layer. anybody have any suggestions on what they would use?