I should have added that there are 3 planted tanks: 2 are using pressurized CO2, 1 is not (lower-light, but use Excel). I use EI all all three, and do weekly 50% water changes.

Actually, it's the "lower-light" tank (15 gallon) that had the lowest pH! And I add the same amount of GH Booster, proportionally that is, to each tank after the water change.

And I think the telling aspect was that in the "hard water premix" (5 gallon tap water + 4 grams of Barr GH Booster) the pH is down around pH ~4.5.