First off, I have never heard of a fantail angel in the SW
trade. I did a quick google and came up with this:
Is it possible you have a filefish? If so they are not reef safe and should go back.
Either way if it is an angel and is nipping it will probably always nip. Unfortunately that is the way of angels, some nip and others do not.
If you have a pic that would be great as I am curious what a fantail angel looks like.