I would like to know what kind of adverse effects high levels of co2 has on fish. An article on the kirb said that some fish can stand levels of 800ppm. Please, someone tell me if that is true, because I am going to lower the ph of my aquarium. The problem is that I have a kh of 18. So, If I wanted to get to 6.8, then I'd be somewhere around 50ppm, I'd guess. Am I correct, and can fish stand these levels?