I don't know where your getting your advice from but give them a WIDE birth,,,,
If two FX5's can't handle a 240 then I fail to see how substuting two canister filters with a third thier loaded capacity is going to help.
Your options are limited by the planned instilation but i would still recomend a pair of FX5's... You hear quite a bit about them bieng nothing more than huge mechanical filters...and while they DO excell at it...with all three baskets filled with bio-material (I use cerimac rings) they have MORE than enough bio-capacity for a 240....EACH
There IS one issue...I recomend (if at all possible) putting one FX5 at each end of the tank...Normally I'd say put both intakes on one side and both outlets on the other but the hose length becomes a problem...even if you use PCV pipes the head pressure will exceed what those cannister filters are designed for and reduce the water flow to almost nothing.
If you only have a single end to work with then you'll need a good powerhead or two for extra cerculation.