A fresh water dip for a fish would be terribly stressful, if not lethal. Remember they are constantly drinking and even with a dip, they will bring some freshwater into their system that will not be immediately released - the least result would be potential organ damage due to cell rupture.
Quarantine as Sponger0 said - 4 to 6 weeks in another tank to observe and eliminate the possibility that they will introduce a disease to your display tank. If you don't have a QT tank and can't afford to set one up, find someone who does. This is the thread you want for further details on acclimation. This is the one that deals with tank set-up; there's QT info inside as well as some other very good tips.