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Need a bit more info about the tank the tang is in and how it was treated previously for the ich and if that's gone.
Typically secondary infection and other maladies can be common after treating a fish for a parasitic problem. With fins it will moreso be water quality and /or the diet of the fish. Increased water changes with well aged SW
and feedings with good vitamin soaked foods are usually a good step but will depend on what's been going on with the tang up to now.