Another thing:

RO water with only CalMag in it has very little "ionic strength" to it. In other words, one drop of pH-up or pH-down will have a profound effect on your readings. There's no chemical buffer there to slow down the swings. Doesn't take much to tip the balance.

Get yourself some nice hard tapwater and A/B it against how your RO is acting with your pH adjustments. Dissolved minerals make the tapwater seem more "stable."

Or, try pH balancing distilled water. :thumbsup: (The less dissolved solids you have, the more wildly your solution will react.) HTH, Hermie