It does not sound too good. As you can see from the link below they have to have at least a 125 gallon tank. The stress from being in a smaller tank might be a contributing factor to stress affecting the fish. Hopefully by QT the fish he might get better. Keep an eye out on your water parameters for the QT. You might have to do frequent PWC
`s to keep the water in pristine conditions. They are herbivores and need plenty of dried seaweed or nori. If he does come around start feeding him that or maybe even some spirulina flakes. I hope he makes it.
Saltwater Aquarium Fish for Marine Aquariums: Blue Tang