The Aqua Medic CO2 reactor does not dispense CO2 as a gas bubble into the water. It is rather diffused through the membrane tube directly into the water. The membrane tube consists of a special material which lets gas but no water through. This acts simultaneously as a check valve. The CO2 gas gets through the tube and dissolves on its surface directly in the water. May be used inside the aquarium or used externally. Single reactor good for aquariums up to 50 gallons and double reactor is good for aquariums up to 100 gallons.