When the drop-checker is immersed in the aquarium, the carbon dioxide will be expelled from the aquarium water into the air space of the drop-checker and will gradually be absorbed into the indicator solution. The carbon dioxide gas will seek an equilibrium point between the indicator solution and the aquarium water. As equilibrium is reached, the color of the indicator solution inside the drop checker changes based on the concentration of CO2 in the aquarium water.
If the indicator solution is green in color, adequate levels of carbon dioxide are present in the aquarium and no action is required. If the solution in the drop checker changes to blue or yellow, then CO2 levels in the aquarium are too low or too high.