Don't know whether you consider this a shot or not, but here it goes.....
, that tang in a 24g with 4 other fish can really get stressed. A stressed fish often means a lowered immune system. A loweredimmune system often means ich, velvet, other disease or whatever it is they say your tang has on his tail.
Strong, happy fish can fight off illness better.
Hopefully you'll take something from this experience. Good luck