Tangs are very aggressive toward any fish that resembles them either in color or body shape. Especially if the tang has been in the tank long enough to establish a territory. I'd take the tang out for a while, rearrange the rock, and give the butterfly time to establish its own territory. You might be able to have them both in the tank that way. One thing I should point out is that a 55 isn't really large enough for long term housing of a yellow tang...you need at least a 75 g tank. Also, due to their agressive nature, tangs are usually the last fish added to the tank. Is returning the butterfly an option?
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