problem with tangs and even the more docile tangs is when u try to add another fish with the same body shape especialy in a smaller tank they can get down right nasty, only advise i can give is keep an eye on it for a few days and hope the tang accepts the flame angel, if after a few days the aggression is still there ur gonna have to make a choise to either keep the flame or tang, did u quarentine the flame before adding? reason i ask is if he wasent u run the risk of the added stress for ich