ive had 405's 305's and the fx5 all on various tank sizes. ive even run marineland emperor 400 pros and the aqualclear 70's.
seeing as how you are asking about canister filters ill stick to that and what i know. for each of those filters i filled with nothing but the foam that came with them and filled the rest of the filter up with the fluval bio media. ive also run each of the canisters with eheim bio media. the fluval fx5 will hold the eheim media with the only concern being the smaller of the round "beeds" may fall through the bottom tray. if you do get the fx5 it filters really well which means as much of a pain it is to clean this canister filter it is a necessity to avoid excessive nitrates. another simple "mod" is to replace the stock return nozzle with a pvc
street 90 to reduce restriction. i think it was a 3/4" fitting but dont quote me. that piece just slides into the rubber fitting at the end of the hose in place of the stock nozzle.
i picked up my fx5 off ebay for 250. not cheap but if you watch all the auctions someone is always selling one for around that price.
its a great filter. moves the water well and does a great job. i read a web site that compared the fx5 to an eheim canister of similar stats. they found the eheim was biologically better filtration but didnt say by how much or it was marginal enough not to mention. the fx5 had better mechanical filtration according to them. again they didnt say by how much. thats why i paired the eheim bio media with the fluval...