For some reason, I need to get my drop checker closer to yellow than green to get any response from the plants. Fish are happy. So, in my experience the 'green' range wasn't enough. I don't understand why, though.
I get the same thing.
The green range could indicate as little as 20 ppm, since each of us has a different perception of what "green" is, and the ppm of CO2 is very sensitive to that color being exactly "right". And, the green range could indicate as much as 45 ppm, for the same reason. Not all plants react the same to various CO2 concentrations, so what is needed to get a vigorous response can be different for different plants.
That seems like a very valid explanation.