Freshwater & Tropical Aquarium Salt
Livebearers, some cichlids and fish from brackish habitats need a bit of salt in their water. Many aquarists find that most tropical fish tolerate and benefit from small amounts of salt (no more than 2 tbsp/5 gal), although there is some debate. Salt does inhibit nitrite uptake, so adding a little to a new aquarium can prevent nitrite poisoning before your tank fully cycles.
The main application of freshwater aquarium salt is to kill parasites (make a solution - around 4 tsp salt/gallon - dissolve, dip afflicted fish for 2-3 minutes, remove immediately if distressed). Salt also replaces sodium and chloride ions that help stressed or sick fish recover quickly. (If in doubt, use stress relievers instead.) Don't use salt with live plants unless product descriptions say otherwise, and avoid use with scaleless fishes like cordydoras.