15ml 4dKH premixed drop checker solution.
When the drop checker is submerged in the aquarium, the carbon dioxide will outgas from the aquarium water into the airspace of the drop checker and will eventually be absorbed into the indicator solution. The carbon dioxide gas will seek a point of equilibrium between the indicator solution and the aquarium water. As equilibrium is achieved, the color of the indicator solution inside of the drop checker will change based on the concentration of CO2 in the aquarium water.
If the indicator solution is green in color, then appropriate levels of carbon dioxide exist in the aquarium, and no action is required. If the solution in the drop checker changes to blue or yellow, then the CO2 levels in the aquarium are too low or too high, respectively.