I use a drop checker from Red Sea. It came with the CO2 kit and is pretty helpful at keeping your CO2 steady. The colors are right on the checker for direct comparison. I know what color I am aiming for and if it is off, it acts as an early warning that something in my tank has changed: CO2 tank empty, forgot to open needle valve after cleaning bubble counter, water level too low so pump making too many bubbles on surface...etc.
The drop checker can be the most accurate way to test CO2 it turns out, if you know how to set it up, as shown by Susan's link using DI or distilled water at a dKH of 4. However most just use their tank water.
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