Tropica's 3-in-1 CO2 diffuser contains both a bubble counter, return valve and diffuser. The bubble counter enables an easy and precise dosage of CO2, as you can easily count the large bubbles in the chamber during the setting itself. The return valve, on the other hand, ensures that the water from the aquarium is not sucked backwards into the hose. With this CO2 diffuser, CO2 utilization is increased many times compared to a regular air stone.