This may be an obvious thing -or not!
I just got a big alert after looking at a problem for way too long. Things that we assume are stable may change.
I have some tanks in the Austin, Texas area and the water has always been very hard/ alkaline with lots of buffer. PH7.8 GH 300 PPM range KH 300 PPM range. We had a pool for a time and changing the PH is nearly impossible so after a few years, I got into not really testing the PH. Once I got away from using the 5 in 1 strips, I rarely even noted PH.
But then a couple months ago, things seemed to almost stop growing in one tank controlled by a CO2/ PH controller. I had found I could only press my African cichlids down from 7.8 to 7.0 before they showed stress. Seemed right and the controller works the CO2 injection so why worry?
Why worry? Because it has all changed!
I now find my tap to be 6.8, my reserve water barrel 6.4 and my CO2 controlled tank is set at 6.6!
I had noted some things that seemed like more CO2 might help and worked it down just a touch.
But if the water from my water changes is 6.4 and I'm controlling it to 6.6, I'm not getting any CO2 to my plants, am I?
Test strips may not be accurate but they would have certainly clued me in to this new situation way quicker!
I might assume there are lots of water companies reviewing their PH in response to the Flint, Michigan follies. So if you have some trouble that might be related to CO2/PH questions, a PH check may be worth the time.
I'm currently waiting overnight to test some water to use that method to test how much CO2 I have but I might suspect there is little to none in my tanks.