The thing with Clown and Niger Triggers is they get big. 19"+ in the wild. Your 55 won't be big enough, IMHO. Bluechins (aka Bluethroat) are considered "reef-safe" and grow to about 8"+ wild, maybe 6" captive so that would be a better choice, but still....I don't know about puttin' one in a 55. Your call.
If you do get one, the smaller the better at the start. It will be more likely to be reef-safe that way.
HTH and cheers
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