I have ph controller and I measure my change water which I find 8 PH. I also check the waste water from my aquarium's water change and I found it again 8 PH. But when I inject CO2, I bring the PH to 6.2 before the light are on and my drop checker is lime green and no fishes gasping for air. With the chart is logical, but with 1 PH point drop, is far away.
My ph controller is calibrated and is new.