I've got two canisters on my 75 gal, and my sev seems to be just fine with it. One end of the tank is more calm, and he'll go there overnight (I can see him hiding in the floating water sprite in the mornings at that end of the tank), but the rest of the time he's all over the tank, in and out of the more powerful waterflow areas. Not that they are terribly powerful, but more so than just an HOB or whatever would be.