how much co2 is too much

I've seen in some places that CO2 levels above about 15-20ppm can be harmful
to fish but I've also seen that you can go much higher without harming fish
as long as the O2 content is adequate. I'm asking because I have liquid
rock coming out of the tap, the KH is 15 degrees. I use a pH controller and
I have it set for 7.5. I am getting ready to start stocking with fish and I
want to be adding tetras and corys and other fish that prefer low pH. How
high can I bring the CO2 without it being a problem for the fish?