I use Biomax in the bottom and middle tray then three bags (one box worth) of Zeo-Carb in the top one. Will be changing the top tray to something different shortly though, i only used Zeo-Carb in the top because the tank and filter was a new set up.
I would highly recommend the FX5 as a filter ... I was looking into Ehiem filters but due to the price i thought i'd give the fluval FX5 a try first and i'm delighted with it
I use Bio Max. I have my smaller Fluvals filled with Eheim Substrat Pro. I like the Eheim Bio and used it in the Fx5 before, but some of the spheres are too small for the Fx5 and I've heard of people getting the little spheres stuck in the impeller. I also use Ammonia Remover in one of the trays. I actually have 2 Fx5 set up on one tank and one set up is all Bio Max while the other is a course filter pad and 2 trays of Ammonia Remover.
Save $$$.............. get bulk ceramic rings, they work just as well. Seriously. I have substrat PRO in 3 of my FX5's and the others use ceramic rings. Both work exactly the same.. Get some laundry or media bags to make cleaning easier.