I would treat all fish which were in the same tank as your sick fish. I would also separate the inverts (frags, CUC, etc) from the fish and put them into a different tank (anywhere but the DT).
Also if you are sure it's marine velvet, a fresh water bath before your treatment may reduce number of parasites on a fish and provide temporarily relieve to the fish, depends on the strain of marine velvet.
Good luck! It could be worse (if you put the sick fish in the DT).
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