Most drop checkers take 2 or more hours to show the right color corresponding to the amount of CO2 in the water. That is a major problem if you try to use it to make changes an hour apart. Even when it does turn green about all you can say from that is that you finally have more than 10-15 ppm of CO2 in the water. If you want the full effect of it, then you need to start making very slight increases in bubble rate every day or so, while keeping an eye on the fish for signs of distress. Once you see the plants growing much better, or pearling, you probably now have a good level in the tank.