Check the listing for details. Fish Tank CO2 Bubble Counter Aluminum Non-Return Check Valve.. Color: Option B, Condition: New. Listed at 16.14 USD. Add 4/5 water in the bubble counter and then tighten the top cover. Plug the CO2 pipe and then tighten the trachea fastening nut. Take out the seal ring and install on the trachea interface of the valve, then tighten the bubble counter.