unless you plan to have some sickly overstocked tank, yes. an fx5 would be great on a 180.
the fx5 is capable of more than its given credit. the amount of biomedia it can hold makes it able to support large loads.
my fx5 was supporting my 250g by itself for a few months. that tank had 2 20"+ pacu, a 12" oscar, 10" CK and a few other 5-6" fish. water was always crystal clear, altho the filter foam did gunk up fairly fast, needing to be cleaned every 2 weeks.
I just setup a 180gallon tank with CA/SA and it works like a champ the tank is crystal clear and the fish love the new setup. Right now the tank is pretty heavly stock with about 25cichlids and 2 plecos but I do intend to add another FX5 down the road . Hope it helps I'll try to get some pictures!
I have a FX5, a 40 gallon wet dry sump, 2 X-large sponge filters, 2 Emperor 400's on a 180. It's stocked with 2 large aro's, a large redtail cat, a flowerhorn, and a pleco. I still don't think I have enough filtration. I think a FX5 would be good on a 180, just keep the load very light. I'm selling my redtail cat because he ****s like hell.
Have you done the math or atleast test to make sure that 40 gallon sump will hold the water siphoned into during a power shut down? I can't remember my figures but it seems small.
The only fish I have at the moment is an RTC... I will be adding fish when I get it completely set up. But I would think that this filter will be enough