I am quite new to all this, so feel free to judge my observations with that in mind.
I had real problems with pH in the first few weeks of setting up my marine tank - well I thought I had problems with the pH, what I really had problems with was reading the pH test kit results.
Test kit 1 = Red Sea Marine Lab pH results gave a colour which didnt match anything on the result chart - ever, still doesnt. We had people over one weekend and all eight of us agreed that the colour in the tube just didnt look remotely like anything on the chart. This test kit was completely unreadable for me.
Test kit 2 - Done by the LFS
(sorry no idea what test he used) showed the pH at 7.7. He did say that the pH of the sample would not be the same as the pH of the tank, since the sample had been in a bottle with no water circulation for some time prior to testing.
Test kit 3 = eSHa Aqua Test (dip stick) cheap, cheap, cheap and I got what I paid for! All the colours of the different dip stick test pads ran into each other so not one of them matched anything on the test chart
. Poor test design and poor testing procedure on my part.
Test kit 4 = (I threw the box away, so can't find the brand name, I will hunt it out) was really easy to use and really easy to read ie different pH levels have completely different colours instead of 100 shades of green, meaning no debate about results. It showed a tank pH of 7.9.
I used Seachem buffer once, altered my powerhead to increase surface flow and my pH has settled at 8.3 since then with no fluctuation.
My only other observation on pH was an experiment we did on some RO
water we had in a stored in a sealed container for a couple of days. We opened the container and tested the pH = 7.7 then rocked the container back and forth for 5 minutes, retested pH = 8.1, quite a difference when the only changing parameter was water movement.
Goodluck in solving this mystery, I'm sure you will get there.