I have tried 2 drop checkers numerous times, with various KH fluids in them, hoping that would improve the accuracy. It never did. The basic problem is the difficulty in judging the color of the solution - a slight mistake in judging what is "green" makes a big mistake in the ppm of CO2 you think you have. I ended up the happiest when I used yellow as the desired color, and DIY CO2 so I could be sure I wouldn't ever have too much CO2. Then I just tried to keep the DC color yellow, which was relatively easy.