The other side of too much, is not enough...I have seen quite a few posts on this board from planted tank DIY Co2 users who have killed their fish by not having enough surface agitation (which releases Co2 needed by the plants). The amount of aeration/surface agitation depends on your stocking, temperature, and the shape of your tank...more fish need more oxygen, hotter water temps. carry less oxygen, long tanks with a lot of surface area on the top need less agitation than comporable sizes of the tall tanks. I just figure it's better to be safe than sorry. For most tanks, HOB filters create the needed agitation. Canister filters also oxygenate the water. I know I have too much, but everyone is alive...including a few plants. In my 125, I have a powerhead in each corner - not running the air, but pointed toward the surface to agitate. I suppose I use these more to move the heated water away from the heaters I have in each back corner of the tank. Second, my canister filter return is pointed up to roil the surface. And last, I have an 18 inch bubble bar along the back - totally unecessary at this point, but it's for decoration (can't stand a bare back wall) and my Pleco loves to swim and lay in it. At some point, I will up my light wattage and plant more heavily...then the bubble bar will go. But I will be keeping my powerheads even if it kills my plants. It's a fish tank first, with plants second.
