My 29 gallon tank has 1 tablespoon of aquarium salt in it. I initially added it because I had guppies and mollies in my tank, and a pleco and rainbow shark. My guppies/mollies are gone now (some died, others were given away), but I kept the salt, because my pleco and shark were used to the water being salted. I now have 2 cardinal tetras, 4 tiger barbs, the pleco and shark, and they're all doing fine.
My fry tank (when I used it) was a 6 gallon, and I had 1 1/2 teaspoons in there, and higher heat (80 deg.). The fry all thrived in that tank. That tank is running empty now, as a hospital tank, and has 2 teaspoons in it. Salt and extra heat I find help any sick fishie feel better...
~Tara