salt may be good for the tank if there IS an infection. If there isn't, using salt as a preventative IMO is a bad idea. For instance, plecos. Plecos are fine with salt if used temporarily for curing something. If it is used long term, you will notice this scaleless fish begin to fade in color and eventually die.
I realize that and am not for the use of salt as a preventative. I was referring to his statement that at his store he sees a difference when they add salt vs. not. At my store, I've never seen a difference. There are still dead fish in the tanks when I start my shift every morning.