You can but depending on stocking levels it might not be enough,I have a ac110 and an fx5 on a 125 gallon and i have never had any problems ,even when it was heavily stocked.
All depends on your stocking. If you go a route like me with 2 oscars then you woudl need 2 canisters to keep up. I am running an FX5 and an XP4 and still have to do 2 water changes a week and almost daily poop cleaning..
Africans are not near as dirty as americans. If you can afford an Fx5 go for it! I have 2 (on my 180) and I love them. They'll polish your water and hold tons of bio for many africans. I believe that one Fx5 will be just fine for your 125.