Peat moss isn't soil. As noted, if not amended with lime, the ph will drop alot as the peat moss breaks down. Worm castings would also allow the ph to be stabilized and not allow it to drop drastically. If you were growing in actual soil, you wouldn't be in danger of ph dropping too much too fast (provided feedings aren't crazy acidic). What exactly are you growing in? People consider peat moss (which many bags of dirt are) as soil, which it's not. If using RO water, yes, you need to add cal-mag, but that yellowing is neither of those deficiencies..all signs point to sulfur. Does your nute contain sulfur? I know for some reason, many 'regular' commercial plant foods don't seem to. The yellowing veins at the top and green between veins, not to mention the overall bleaching of the top, seems like a sure sulfur deficiency. If your ph was the only reason causing this, then other elements would not be getting through either. Meaning, if ph was too low, you would see calcium and magnesium issues popping up, with possibly potassium. If it was too high, you'd see a host of other problems with the top as heavier micros (iron, copper, zinc, manganese) get locked out. How much water did you allow to run through when you took that runoff reading?