As to the "why" question, personally I think the clown tang has a similar shape to the powder brown.
powder brown vs clown tang
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The clown tang may establish himself enough to fight back once you put the powder brown back in, maybe. Assuming the clown tang hasn't already kicked the bucket. Funny, I see LFS bring in clown tangs ALL the time and they are beautiful fish, but I have never seen a clown tang in any hobbyists tank.. poor little guys, they really do belong in the sea probably.