Dangerous pH levels for the fish are lower than I suspect you think. As long as you are lowering the pH with CO2, and not by adding muriatic acid or other acids, the fish aren't going to be bothered by some pretty low pH's. But, adding too much CO2 will bother the fish by suffocating them. pH just isn't as critical a parameter for them as most of us used to think it was, but total dissolved solids is probably more critical than once thought. That's why changing 80-90% of the tank water with tap water doesn't normally bother the fish at all, even though the pH will likely change quite a bit while doing that, but adding a big measure of bicarbonate of soda to raise the KH will bother the fish. At least, that is how I am interpreting what I have been reading here and on other forums.