[ray-the-pilot]

I have a pH controller and can regulate the level of CO2 in my tank very accurately and my experience is that even low levels of CO2 have negative effects on some fish.

I keep my CO2 below 20 ppm, which is dark green-blue green on a drop checker. Even at this level I can see that some fish (hatchet fish, cardinal tetras, oto cats) show a decline in activity and reproductive interest.

If you are interested in fish health, I would never let a drop checker get light green.