Thing to realise is that pH changes do NOT affect fish. I'll say that again, because it goes against so much that you read and hear, but pH changes do NOT affect fish. Changes in TDS - total dissolved solids - which are often associated with pH changes do affect fish, potentially fatally, but changes in pH, with a range from around 5.5 to 9, do NOT affect fish. Rams do not like a pH below 6; they like the sort of TDS which is generally associated with a very low pH, but super soft water at pH 8 would suit them fine.
Consequently, the pH changes associated with CO2 addition, which are not associated with a TDS change, do not bother the fish - they don't even notice. Bunging chemicals in to change the pH back to normal, which do change TDS, will stress them.
The role of KH in CO2 injection is misunderstood. It does not reduce or prevent the pH drop, it just changes the start and therefore the end point of the shift. Moreover, KH drops over time between water changes, so if it's low from the tap it'll be even lower when water change time comes. Very low pHs (like below 6) will inhibit filter bacteria, so water which is in danger of dropping very low because of low KH is buffered to a KH of around 4 to prevent this from occurring. Me, I'm not convinced it's necessary; my tank drops below pH 6 with CO2 because the KH is 4 out of the tap and I filter over peat to lower the GH of 8; this also lowers the pH significantly to the degree that it's 6.4 without CO2 injection, so probably around the mid 5s with; fish fine and breeding, biological filtration probably minimal but the whole point is the plants are using the ammonia anyway. Besides, at that pH you'd need bottlefuls of the stuff to get enough toxic NH3.
Consequently, the pH changes associated with CO2 addition, which are not associated with a TDS change, do not bother the fish - they don't even notice. Bunging chemicals in to change the pH back to normal, which do change TDS, will stress them.
The role of KH in CO2 injection is misunderstood. It does not reduce or prevent the pH drop, it just changes the start and therefore the end point of the shift. Moreover, KH drops over time between water changes, so if it's low from the tap it'll be even lower when water change time comes. Very low pHs (like below 6) will inhibit filter bacteria, so water which is in danger of dropping very low because of low KH is buffered to a KH of around 4 to prevent this from occurring. Me, I'm not convinced it's necessary; my tank drops below pH 6 with CO2 because the KH is 4 out of the tap and I filter over peat to lower the GH of 8; this also lowers the pH significantly to the degree that it's 6.4 without CO2 injection, so probably around the mid 5s with; fish fine and breeding, biological filtration probably minimal but the whole point is the plants are using the ammonia anyway. Besides, at that pH you'd need bottlefuls of the stuff to get enough toxic NH3.
