If it still want to eat, hand feed it.
Give Henry's gourmet which contain all the formula to make a sick fish dance.
About 2 weeks ago, my filefish has a ugly lump on the left side, either could be cancerous or bacteria infection, and it was 45deg swimming at its sides. I thought it was going to die in a few days time.
Doesn't want to eat but I throw Henry gourmet everyday in the tank, slowly it start to eat. Now, even though the lump is getting smaller, after the lump broke and expose the insides, it is eating like my other tangs. Now much healthier than before and hunting for aiptasia during its spare time.