I use a drop checker but I've found that, at least to my eye, it stays green over a wide range. I initially wanted to target 30 ppm and I did this by measuring the pH change of a small amount of tank water that has sat out in a paper cup for 24 hours compared to that inside the tank. A difference of 1 degree is supposed to equal 30 ppm. I raised the CO2 up 1.4 degrees chasing some algae, and the drop checker still looked green.
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