You don't need to drop the salinity that far. 20-21 ppt would do just fine for easing the stress of this fish. It will not cure the problem either. The problem could be from several sources; damage from netting at the LFS, an internal bacterial infection or blockage. The swim bladder could be damaged or pressure from blockage/infection are not allowing it to function properly.
Since we cannot make any assumptions as to which one, we need to treat for the ones we can. So a proper gram negative antibiotic will be needed. Go to the LFS and see if they have either Maracyn II for SW (Minocycline @2mg/gal first day and 1mg/gal afterwards for 10 days), Gentamicin (20mg/gal for 7 days) or Kanamycin (190 mg/gal added every 3rd day for 3 treatments) and purchase an appropriate amount for your QT size. If this is a dietary problem and the fish is still accepting foods, get some frozen peas and make sure they're cooked/soft before using them. Mash an appropriate amount in with the angels food and see if it will take to it. The peas will "grease the wheels" so to speak and help pass the blockage. We will also be added some of the med to the food so be sure to post back with whichever you get. The biggest contributor for blockage in a fish like this is a mainly meat diet. Once the diet is corrected, the fish will recover quite quickly.
I will warn you though if the bladder has been damaged, it may not be a good outcome. Was the fish easily caught at the LFS or difficult? At any time was in pinned?
How did you acclimate the angel?