There are two types of Christmas Wrasse and you need to make the distinction. One is HUGE and not reef safe (I do not suggest it be kept)...the other is smaller and reef safe.
I've posted this before (check the names in fishbase to determine what you want):
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There are two species of wrasse in the hobby often referred to as the "Christmas wrasse" and it is extremely important to distinguish them. Thalassoma trilobatum is actually the "true" Christmas Wrasse and it gets huge (a foot or so) and is definitely not reef safe. Halichoeres ornatissimus is the reef safe variety, and is often more commonly referred to as the "ornate wrasse." It is more reef safe than the larger guy, but still a potential risk to smaller crustaceans and stuff.