The issue isn't that the well water has a pH problem, which is why I didn't catch it right away. The pH is in the high 6's, no problems there... the issue is the lack of dissolved minerals. I tested it for dissolved lime and came up with almost NONE!!! But while I was doing that test, I found a SHITLOAD of buffering in the range 6.2-6.6, indicating high levels of organic acids. What this means is that the lime in my promix is getting used up WAY faster than it should be considering the pH level is correct, and the plants are showing signs of pH problems in their last 2-3 weeks of flower. So my ProMix is going bad 5-6 weeks after first wetting. Now I am treating my well water like distilled, and adding calmag plus to slow this down.

This is an instruction for that analysis:
Massachusetts Water Watch Partnership pH/ANC Protocols
You need a GOOD sensitive pH meter, a Hach digital titrator, and a Hach sulfuric acid .16N cartrige. Also labware- a source of clean deionized or distilled water, clean beaker, and a magnetic stir plate is a good thing to have too.