You may be fighting a losing battle. If the fish was collected with chemicals, and was exposed to too much cyanide, your tang may not be able to digest the food that it eats.
This is why you should only purchase fish that are healthy in the LFS. Feeling empathy for sick fish is certainly understandable, but the collection and shipping process sometimes is so hard on them that they are unsaveable by the time that they reach the LFS...
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Having said that, make sure that there is always some seaweed available. (wrapped around a piece of PVC tubing, held on by a rubber band and suspended by a string is what I do) - also, supplement with daily feedings of mysis.
Fish are grazers, and will eat a little bit at a time, all day long - not a lot of food at one time.
Good luck with yours
Mitch