- How to install your CO2 Drop Checker Invert your drop checker and fill the reservoir up half way with CO2 drop checker indicator solution. Using the suction cup, submerge the drop checker into your aquarium water and stick to the aquarium glass sides. Based on the concentration of your CO2 in your water, the colour of the solution will change. Check the colour. Blue = too little CO2 concentration, Yellow = too much CO2 (dangerous levels for fish), Green = optimum levels for plants and fish. The colour has a 1-2 hour delay. So what you are reading is the CO2 concentration roughly 1-2 hours ago. Based on your reading, adjust your CO2 bubble rate using your regulator if required. Wait a further 2 hours for concentration of CO2 to change and check your readings.