I don't understand why shutting down one of the filters helped the CO2 issue, unless it was disturbing the water surface too much. A powerhead would do the same unless you carefully aim the flow so it only lightly moves the surface water. The size of the powerhead is not determined by the rated size - that is referring to using the powerhead for it's original purpose, to drive an undergravel filter. Go by the gallons per hour number instead. I use a 120 gph powerhead in my 45 gallon tank and it moves the water very well for me, so I suspect a 200 gph unit would be adequate for circulating your water.