I have an Ultima II 1000 on our 600 gallon FRT tank. I think it would do perfectly fine by itself, if I had a moderately stocked tank with fish only. But, with the turtle, I have to backwash the filter every 3-4 days to keep the tank crystal clear. And, when I backwash the filter, I use about 150-200 gallons to do two cycles of "Backwash" & "Rinse" to get my pvc sight tube clear.
Do you feel that one of the larger models would not clog as quickly? My 1000's will run off of a pump inside the tank mounted mid tank, on the back wall. The 1000 is biologically adequate bit I feel like mechanical suffers if forced into a smaller diameter filter.