Have you checked your KH as i had a similar problem in my Discus tank recently.
All my fish were ok, but it was new cardinals i added that were in a bad way not long after adding them, which made me check my water stats.
My Ph was always around 6.6/6.8 but this time it was only 6 when i tested, and i don,t know if it ever dropped lower than this.
My KH was only 1 and i don,t know how long my Ph was unstable as i think it must have been very erratic with such a low KH reading.
I added crushed oyster shell to one of my externals that has brought the KH up to 4, GH 6, and Ph is consistant at 6.8
I,ve used PN and Easycarbo and didn,t find it altered my Ph in the past.