My tap water has always had a PH around 7.5. But lately my tanks have been
running over 8, where they used to be around 7.6 - 7.8 in the past. So I
tested the KH (5) and PH (7.0) of my tap water, which would seem to indicate
15 ppm of CO2. I'm a little surprised to find it this high during the summer
(warmer water). I'm letting a batch of tap water sit, so I can test the PH
after the CO2 escapes.

I don't know what the KH was in the past, the city water documents no longer
list KH or DH.