According to my drop checker I have occasionallly gone to 40+ ppm of CO2, and the effect was that the fish clustered at the surface - not all of them, but most of them. So, I think it takes more than 40 ppm to kill fish in a brief time. The pH controller will work if you first determine that you have 30 ppm of CO2 in the water, using the drop checker method. Then, measure the tank water pH, and set the controller to maintain that pH. If conditions in the tank change you have to repeat this to maintain the same accuracy.