Normally the ph drop is about 1 point with co2 but theat is over a period of time,speaking only of non Discus tanks there is no harm in changing water in a co2 filled tank as the drop is only minimal have two hi tech tanks 40 and 45 with full pressurized co2 and for the last 6 yrs. have had no issue with that situation.As for a Discus tank I was talked out of co2[and happy I was] as a precaution and just to keep things as simple as possible but I see no problem with a slight ph drop.When changing water in a large tank the drop is probably going to be closer to .5 of a point...