If you have no plants or are not injecting CO2, the bio-wheel is IMO great, but if you already own a canister there are plenty of ways to enhance the gas exchange at your surface. And from what I have read so far you will be hard pressed to find a canister owner who would say anything bad about them. As a gas excnage is pretty easy, IME oxygen defeciencies usually relate more to circulation or current. A lot more info would make answering much easier i.e. tank size filter size, plants fish load, which fish at top, which are swimming normal, do you have any surface ripples etc. You may just have two fish who like to look around at the top.