started to use the exhaust fan during the night to control humidity for it was 60% at night. bright it down to 35. had ph pretty low on the hydro plant around 5.5 and was noticing her unhappy so i tried to bring it up.
i concur with your diagnosis -- ph fluctuation
if it continues and spreads watch the pattern sometimes light burn starts between the ribs of the leaf like that as yellow and spreads
especially if it gets suspiciously even patterning -- ph spots are usually irregular spots
@DojoGeejer, never used it for that purpose but i am a big fan of potassium silicate in coir, makes much stronger plants
it will definitely make the roots of any of your plants less susceptible to shock from any source so it should work to help against ph shock i suppose?