Hey Andy welcome to SD !
I breed rams and discus when they want to cooperate[the discus] ..
I would start testing TDS instead of pH.
IMO pH means very little compared to TDS.
I also [maybe just me] don't sweat low KH readings .
I have never had a swing due to KH and my tap is 2-3 KH and some of my fish go in almost straight RO which I only test TDS on .
My RO lowers my TDS from 350+ to 17 but does not move my pH more then .5.It goes from 7.6 to 7.1 or so ..
I have found the pH matters very little and is much harder to control then TDS.
When I offer the same fish lower TDS water with very close to the same pH it makes a huge difference .
I believe in TDS so much I test nothing else anymore ??
I use the HM3 TDS meter which is easily found under $20 and needs no calibration .
https://www.amazon.com/HM-Digital-TD.../dp/B000VTQM70