Could be a blockage or possibley an internal infection. I would suggest a large water change with properly mixed SW
that has had a day or two to aggitate. Make sure you check temp, ph alk
and salinity to match up with the display tank. Feed the softened pea/shrimp mixture and see if that helps. Might mix in a bit of garlic extract to see if you can get a better feeding response. Fox's are primarily herbavors so diet may have alot to do with it or it could be netting damage or aggression from the tang.
If that does not yield possitive results you may need to QT
and treat with an antibiotic. For internal problems nitrofurazone will usually help the best.
On the subject of the two larger fish though, be aware that these two should not be housed in smaller tanks. Both will get quite large and the tank should be upgraded in size as soon as possible.