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
DreadedHermie Reviewed by DreadedHermie on . RO H20 + Cal Mag Plus = Unstable pH Hello farmers having some pH problems. Here's the setup... 20 gallon tote with 6 airstones for a bubbling DWC for seedlings. Inside the 20 gallon tote there is currently 15 gallons RO H20 w/ 75 mL of Cal Mag Plus (5 mL per gallon Cal Mag Plus). There is nothing else in this reservoir. Air temps are around 68 degrees farenheit and water temps are between 63 - 68 degrees farenheit. Rating: 5