Co2 drop checkers provide a quick and easy method of monitoring the levels of dissolved co2 within aquarium water. A drop checker should be filled 3/4 full with 4dkh solution and co2 indicator solution. As the co2 comes into contact with the drop checker, the ph of the solution is altered, resulting in a colour change. It is this colour change that gives a rough estimate as to the amount of co2 in the aquarium water. Blue being too little, lime green being just right and yellow being too much. As aquarium and tap water may already contain sources of acidity, it is essential to use 4dkh solution to ensure reliable results.