Well, CO2 doe sNOT change that much at lower ppm's, it changes a lot at the higher ranges. When folks belly ache about not being able to add CO2 because it gases their fish at above 20-30ppm, it's because they are winging it and not being careful, going slow and watching.
Perhaps their O2 is poor, perhaps there's surface scum, perhaps the CO2 equipment is hard to adjust precisely and accurately.
Damn, there's 101 ways to screw CO2 up.
This is more an issue for folks trying to tweak things, GO SLOWLY.
As you increase the CO2, you also decrease the changes you make and a SMALL change in pH = a huge change in CO2.
I see this in my 120, the 180 has a higher buffer of CO2.
A change of 0.2 pH units will result in glass algae and hair algae in the 120.
But that's about 20 ppm worth of CO2!!!!
Which is a lot frankly.
I keep trying different ways to explain this, I have done it 101 ways so far, and I'll think of more at some point also.
People really have a simple understanding of CO2, which is unfortunate.
Hopefully over time, I can change that view.