I was less commenting on the PH you have but instead the difference between what i consider you very low PH (and border line salinity as well) and that of the LFS
where you bought the fish. Given that it's been weeks it's probably not germane but it still must be an acclimation shock.
With PH , it depends on when you take the readings, if you have an open tank or lids, etc. Always take the reading at the same time of day, preferably after lights out (I believe). I haven't tested for PH in so long I can't remember for sure