S.T. International Aquarium CO2 Hanging Bubble Counter with Check Valve CO2 Hanging Bubble Counter with Check Valve PREVENTS BACK FLOW into the cylinder with it's REFINED aesthetic style. Made with Acrylic, versus plastic like many other brands, makes it extremely durable and prevents the bubble counter from getting clouded. It's TRANSPARENT design makes for EASY observations.This product may be used with a regulator to depressurize the system. EFFECTIVELY calculates the CO2 quantity being released into the aquatic tank when used with a CO2 regulator. This unique CO2 Bubble Counter with Check Valve provides DOUBLE PROTECTION for your CO2 pressure regulator, CO2 cylinders and other accessories. Does NOT waste CO2!