A CO2 Brass Bubble Counter with Check Valve is a device used in aquariums to measure the rate of carbon dioxide (CO2) injection into the tank. It consists of a clear plastic tube with a brass counterweight and check valve at the end. The check valve prevents backflow of CO2 from the tank, and the counterweight makes it easy to count bubbles as they pass. This helps the user accurately measure the amount of CO2 being injected into the tank, allowing for more precise control of the aquarium's environment.