If the tank is not planted (ie, no CO2 injection) and you're after maximum flow rate and mechanical filtration, the 2 HOB's with 1 canister is good. I ran 2 canisters with a HOB for a while on my 125 (AC500, XP3, Eheim 2028), but have since replaced the XP3 and AC with an FX5 because I am injecting CO2. If you want more flow rate and only want canisters, considering coupling the XP3 with an FX5. I am very happy with mine.