Well, I finally got my second canister for the tank (a Fluval 405) as I was happy with my original one, it is extremely quiet, and supplies are plentiful for Fluvals here. Anywho, I plan on eventually phasing out at least one of my HOB Emperors as it gets louder with time. How much surface agitation is required for oxygen exchange without HOB's? (especially a tank with few plants) I don't have any airstones currently but some of the powerheads and canister outflows can be positioned near the surface.