3% is 30 ppt, or just about what sea water is. I don't see what a dip would do to kill parasites. If the fish lives through it, I'll bet so would the bad guys. It may be like chicken soup, couldn't hurt.

If you have no existing population, I would just add them to the system. If you have an existing population, isolate them in a separate system long enough to see if they are sick. Commercial breeders live & die by their rep, so they are squeaky clean when it comes to this stuff. Pet stores sell to rubes. Breeders sell to business people.