External CO2 Reactor. For Aquariums up to 500 gallons. The reactor 1000 is a hermetically sealed unit for introducing CO2 into larger aquariums. It can be mounted outside the aquarium or inside the stand. The reactor 1000 is supplied with a strong holder. Dimensions - total height is 15 inches with a diameter of 3 inches. Hose connections: 1/2 inch input & output. Recommended flow rate of 250 gph.