I would put it in a quarantine tank if you haven't already and get the salinity up to 3% for starters. Do the standard one teaspoon per gallon for first day, repeat 2nd and 3rd day - that will get you up to 3%. I'm not well versed on ph, but I think it's weird that your ph is low like that. What is your ph out of the tap? You said water parameters are fine, but even still I would do at least a 50% water change in the main tank. Consider vacuuming the substrate if you have any and cleaning out your filter of accumulated debris.