IMO the mechanical stage of the FX5 is fine without any extra help so I just fill the baskets with low clogging bio-media. (ceramic rings...a LOT of them)
Like BIG...This is never the only filtration I run. I like a multi filter approch.
I use modifyed E400's for my main bio-filtration....cannisters for pressurised mechanical filtration and hydro5 sponge filters for safety.