If you aerate your water for a day or two before you use it PH will be more stable in the reservoir to a point. It precipitates some of the calcium carbonate. When the plants transition from veg to flower the roots will exude substances that will drop the PH like a rock. In the soil this action feeds the micro organisms that provide potassium to the plant stimulating them to provide more - cool huh. Not good in our reservoirs though. This will happen for a couple of days and stop. This is your clue to shift to full on flower nutrients. In autopots if you have big swings you should try mixing your nutrients a few days ahead with aeration then a final PH before going into the res.