Ah, this topic is debatable. I've heard of instances where CO2 was over 100ppm. No fish died. Then I've heard where it has got to only 40ppm and the tank was wiped out (just fish). Though my experience and reading, it seems to me the CO2 concentration has nothing to do with fish dieing. Its the SUDDEN and LARGE pH swing that comes with overdosing CO2 that kills. The reason why some one was able to keep fish in water with over 100ppm was probably due to a very slow increase in pH, giving the fish time to adjust.
Anyway, 20ppm of CO2 is good. There really isn't any reason to go higher than 30ppm.