Interesting, you have issues with water PH changes also.....Mine varies anywhere from 5 to7.5 after a 24 hour sit. I RO to remove iron, so no junk to worry about. I find that my changes occur the greatest when the weather dumps a bunch of rain. I do not know why and I have not talked to the city as no need to enact red flags.

I have in the past damaged my plants from these changes. BUT, the good old drop test kit keeps me in the safe range and costs a minimal amount. Those fancy PH things concern me as to how long they may or may not function properly. I have adapted to the drop kit and it works for me.....I am such a fan of the KISS methodology too.