JBL Proflora Co2 Count Safe 2Bubble counter with backflow stop. Key Features: To adjust the CO2 level: bubble counter with backflow stop for the exact dosing of the CO2 supply in the aquarium. Fill bubble counter with water, connect with hose to CO2 system and diffuser, fix outside the aquarium with 2 suction cups on the glass or 2 screws on the furniture. Easy bubble check: CO2 bubbles are visible and countable, no backflow thanks to check valve, for all common CO2 systems.