This test solution is added directly to a CO2 Drop Checker.  Over a period of a few hours the colour of the solution changes to indicate the concentration of CO2 in the water at the time; blue indicates a lack of CO2, yellow indicates too much CO2, and green indicates the desired level of CO2. Depending on the colour of the drop checker you can adjust your CO2 dose until the desired level of CO2 is reached.

The solution comes in a handy 15ml dropper bottle for easy deployment into the drop checker.