When using a drop checker in your aquarium, you should know what the color of the indicator solution means for the CO2 levels in your aquarium. Accurate color observations can be made approximately 2 hours after initial placement of the drop checker in your tank.
BLUE —> too little CO2 or low CO2 levels —> increase the rate of CO2
GREEN —> proper CO2 levels —> no action needed
YELLOW —> too much CO2 or high CO2 levels —> decrease the rate of CO2