I'm not, nor ever will be, a CO2 user, but I don't think you (or your fish) need to be concerned about the changes in pH from adding CO2.Tenfold increase/decrease of acidity over the day
A similar change occurs when the monsoon arrives in those areas affected by it (the origin of many of our Asian fish), and when summer comes to the Himalayas, feeding large amounts of meltwater into many rivers and streams.though at the start of raining season waterlevels can rise 10-20 meter in the amazon which will change watercomposition hugely.