A CO2 drop tester is an essential tool for controlling CO2 levels in the aquarium: the CO2 drop tester is a tank designed to contain an indicator solution (not included) and an airspace Test principle: When the fall tester is immersed in the aquarium, the carbon dioxide will degasses from the aquarium water into the airspace of the fall tester and eventually be absorbed into the indicator solution (not included) Carbon dioxide gas will seek a balance between the indicator solution (not included) and the water in the aquarium. As the balance is achieved, the color of the indicator solution within the drop control will change depending on the concentration of CO2 in the aquarium water. Heavy Duty Suction Cup: Make sure it sticks to the side of the tank Size: the glass drop check is approximately 6 cm / 2.36 inches tall