I'm not sure how to answer you. The point of a CO2 reactor is to get CO2 into the water. It's not a filter. GPH shouldn't even be factored here, within reason. If it takes 80 or 100 GPH or less and dissolves the desired amount of CO2 into the water, what's the problem? Circulating that water is the job of the main filter, and whatever else you have in the tank to facilitate movement.