Freshwater fish in saltwater...

Question
Okay, my question is that if a freshwater fish is introduced into saltwater then back into freshwater will that in anyway harm or drastically effect a fish's longevity or overall health of the fish. I ask this not because i have done it but the well-known Petsmart does. The lady at the store told me they pump a high salinity saltwater into "all" there tanks. Thank you for taking time to read my question and i hope you can help me.
Other info.: I am currently in high school taking zoology as an elective. I hope my love for animals(not just fish)will take me through college as a zoology major.
-Chris Mercer-

Answer
Chris,
it is common practice to dip fresh water fish in saltwater and vice versa to kill parasites, clear fungus etc.
The short answer to the question is it will not hurt the fish long term.
I doubt Petsmart pumps "high salinity" water into their tanks, there is no sound reasoning behind it.
Good luck with your career.