CO2 Drop Checkers are used for long-term monitoring of the CO2 levels in the aquarium. A CO2 reagent is filled into those CO2 Drop Checkers and will change the color according to the amount of dissolved CO2 in the aquarium water. Green will indicate a proper CO2 value, yellow will indicate to much CO2 and blue will show not enough CO2.
The indicator solution turns green at approx. 20-25 mg/l of CO2.