i know that problem. Without RO water, PH fluctuation depends on O2, CO2, and the amount of roots in the bubbler or rez. In the first stage of the plant, few roots, and generally no CO2 added, the PH can go from 5.5 to 7.5 in 48 h. It's due to the amount of O2 in the water provided by your air pomp. So you add acid every day less and the PH goes stabilizing. (If your air pomp is out of order the PH will drop dramatically because no O2 but many cl of acid added). But when you change your water, you introduce new carbonate ions and once again the same reaction. If you add CO2 in the air, it will go in the water, and will lower the PH naturally. When there is a lot of roots, there is less fluctuation but because it's generally in bloom phase i add CO2 and i've to higher the PH adding some new fresh water. In aquariophilie the CO2 is a way to lower the PH. The ideal would be :
Veg stage : CO2 and acid in the water.
Bloom stage : CO2 and no acid in the water
With RO water, no carbonate ions, so less fluctuation with O2, but not with CO2.
That's my mind.