Yet another Tap water warning

[SMB2]

My six month 90 gal set up has been cruising along with a PH of 7.6, recorded with a Milwaukee probe. Tap water always 7.1 that rises to 7.6 with 24 hours of aging. I have a planted tank and amon. nitrite have been 0 for months. Nitrates run 5-10 ppm (my tap is 5ppm). I do a mid week 30% WC and a weekend 80% change with routine filter/sponge maintenance.
Today after an 80% WC my PH was 7.2. I checked the probe and calibrated it, also checked the PH with an API test. The fish looked fine, no heavy breathing etc. So I checked my tap PH (at 82 degrees, same as the tank).
The PH was 6.7. Have no reason for the change and not sure who to ask but as with other posts this argues for checking your tap routinely.
I think I will switch to more frequent small volume WC for a while until I see what is going on with the water supply. So far thankfully the Discus don't seem to have been affected by the acute drop.

[GrayLadyPat]

Ask your water department for an analysis. Most will email you one for free. It should have a graphical representation of what your pH is through the year.

My pH is dependent upon the pH of my local lake. If it fluctuates, so does my tank. In the spring/early summer when the lake is more turbid, my pH crashes for about a month and I have to add crushed coral or some other natural pH booster to the tank, and then take it out when the pH normalizes again. Mine has gone as low as 5.9 during those months, and as high as 8 in the winter. So, as long as the change happens slowly enough, and the fish aren't stressed, just keep an eye on it.

Good luck to you!

[SMB2]

Thanks,
A swing from 7.6 to 7.1 with one water change was more of a drop then I would like and had I picked up on it I would have done smaller water changes. Fish seem fine this morning, battling for their place at the breakfast table as usual.
I did age another 30 gal over night for a QT tank and again the PH is 7.1 (also in the display tank this morning). So I think it is "stable" and not something in my system. It is convenient
running a PH monitor.