Your tank is on the young side for such a fish as this. The fish is possibly having trouble with your water quality. It isn't something you measure necessarily (like ammonia, nitrite, etc.) it's just the fact that your aquariums hasn't stabilized. The pH control being off is an indication of this. Several kinds of fish entering such a young and transitioning system with a low pH will show signs of a bacterial attack, which I think your fish might be facing. Without a photo it is hard to say.
If it is bacterial, the fish should be removed to a hospital tank and treated with an antibiotic. Since so far this might be a topical condition, a good antibiotic would be NITROFURAZONE (in the product, Furan-2). Also another choice would be Maracyn Two for Saltwater fishes at double the recommended initial dose.
Careful about changing the pH. You should raise the pH to 8.2 SLOWLY (like over a 7-day period). Fish don't like sudden even small changes in pH.
Good luck! :rollface: