I add aquarium salt with every water change. The product I use recomends one rounded tablespoon for every U.S. 5 gallons as a general stress reducer or too aid in desease recovery. I add a fraction of that, just some in the palm of my hand for every 20G I change.
Fish actually remove salts with their gills and it adds electrolytes to help them breath better. For a dollar a year in salt, even if the effect is minimal its part of my waterchange routine - no fish desease in ten years or so.
Too much salt can be bad for plants also, I belive that is correct.