OK, this is beginning to make some sense.
The GH of 180 ppm (equates to 10 dGH) and KH of 120 ppm (equates to 6.7 dKH) are not high; this is moderate hardness. A pH in the 7's is exactly what one would expect. Having said that, it is possible that your water authority added "x" for some specific reason and it targets the pH. It might be instructive to check with the water authority tomorrow (or perhaps on their website) to see if anything in the way of treatment is or has occurred, beyond the normal--and by the way, re the ammonia, if chloramine is added along with chlorine, this usually explains these miniscule ammonia readings, they can be ignored.
The buffering capability here is adequate and not excessive, so with normal regular water changes, keeping the filter well cleaned, not overstocking, not overfeeding, and perhaps vacuuming the open areas during the W/C, it should be fairly stable, as I take it it has been until very recently.
Now to the sudden pH fluctuations in the tank. This could be due to the substrate tab; I know you only added one, but I've no idea what is in this tab. Unless a member with more chemistry knowledge than I have comments, I would keep an eye on the pH in the tank, periodic tests, governed by the results, and not add those tabs. The Flourish Tabs do not do this, guaranteed; I have been using them for more than 10 years, and they really benefit sword plants.