without some sickly overstocked tank, 2 fx5 is more than enough.
my 250g is currently being filtered only by an fx5, thanks to my other filter taking a crap on me. i dident buy a new one to see just how the one fx5 would hold up, and needles to say......water is crystal clear, all levels are under control, and fish are still nice and happy.
my 250 has 2 pacu just breaking 20", a 12" oscar, and a clown knife thats around 12". so id say them alone puts a good stocking on my tank that the fx5 does fine filtering.