I think wkndracer my have figured it out:
Glass of water right out of my tap (with the aerator unscrewed, just like it would be with the water changer) gives:
6.2 or lower PH (low as the test will read)
After 12 hours standing on the counter, ph reads 6.8.
Not fond of airstones in the tank all the time, but looks like I need to run one during water changes (and to let it splash in from above the line, not gently enter underwater)