As I understand this from another forum, (someone correct me if I'm wrong) pH can be lowered 2 ways: through CO2 injection or through buffering water. By injecting CO2 it will have little affect on fish sort of speak because you haven't changed the chemical make-up of water. Through buffering, RO units (etc), your changing the chemical make-up of water which can affect fish, but doesn't mean that CO2 was added or increased. Hope I got that right.