If your ph is dropping there is an issue. Usually root rot or your nutes are way to high and the ppm is increasing. This can happen if you don't do daily top ups.
I was describing what I noticed just with tap water and ro water in separate buckets, no plants, no nutes, no ph adjustments, no other variables. I tested this because I found when I was using the ro water (specifically early in veg) the ph would slightly drop over time from where I had adjusted it and with tap water it either stayed the same or slightly rose. After some research just now I think I found some relevant info:
"Because reverse osmosis removes the minerals in water the water will then react with carbon dioxide upon exposure to air to form carbolic acids, thus lowering the pH. The resultant pH will depend on the initial water chemistry. It cannot be predicted. "
and more technical
"When water is passed through a semi permeable membrane using pressure, many organic and inorganic compounds fail to pass through, however gases like Carbon Dioxide make it to the other side. CO2 combines with the free OH- ions in water to form acidic HCO3, while the H+ ions fail to find any substance to interact with as most of the impurities have been removed through the RO process.
CO2 + H2O <--> H+ + HCO3-
As a result, water has a positive balance of H+ ion, lowering its pH and making it acidic. (Again, remember, pH is measure of H+ with a minus. No more chemistry further, we promise!). The greater the amount of CO2 in your water, the greater is the drop in pH level"