This was done did not add to tank and tested
I just put the correct amount of tap safe in some tap water then tested it and came up with 8 so it was not added to tank then tested.
I would thought it should not change but it did
It's also possible that Ph can change from water sitting for a while as it off-gases certain chemicals. As I recall however, something like distilled water will drop in Ph as it sits for awhile as it absorbed carbon from the air and creates carbonic acid.