His explanation for using salt dips to treat various diseases was good too. It's a somewhat nerve wracking thing to do, and I don't suggest doing it when other meds can be used, but I've treated corys and bettas that had advanced states of bacterial infection (finrot) with it (as a last resort) and had great success.
Even corys can take salt for short periods of time when it's used for medicinal purposes, but long term exposure to it seldom does any good and can do a great deal of harm. There is no disease preventative like a well cleaned tank.
I've been using aquarium salt for a few months now with no problems. I was planning on using it permanetly, but I'm not so sure now. My house has a water softner that uses salt , I'm wondering if this would have any affect on my fish?