How much CO2 does it take to kill fish? I am looking around and can't seem to find a clear answer. I imagine there must be a number of variables that could lead to fish death by CO2, not just the level, but find myself a bit ignorant on this topic... Anyone?

Back story - I was just told that a local AZ hobbyist nuked his fish by adjusting his pH down -0.1 from 7.0 to 6.9 in 7dkH (130ppm kH) water... This was MY advice based on an algae problem he was experiencing, needless to say I feel a bit rotten...

He is managing his CO2 with a pH controller. Is this really possible? I think it is a safe assumption that the tank was not healthy before the change, but from what I understand it was stable at 7.0 (20ppm of CO2) - or at least the fish were not dying...

Thoughts?