Aquarium Salt can be used as a medication as it can aid the promotion of a fishes slime coating (which protects them from disease and conserves their energy), it can also relieve stress in fish, help in healing wounds and be used to help eliminate parasites (using a salt bath). It can sometimes help to lower NitrIte. Any salt used should be free of additives such as iodine.
Freshwater fish do not need the addition of salt in their tank or to use it as a preventative measure (if used constantly, parasites etc. become immune to the salt and it becomes ineffective), it should only be used as a medication when the need arises. Salt also doesn’t evaporate, and can only be removed by water changes and plants will not survive higher concentrations as well.
Freshwater fish already contain salt in their body (just as we do) and have to constantly eliminate the water - mainly through respiration and urine, and if there is salt in the water it can be very detrimental to their health and will affect their equilibrium. Whereas saltwater fish have to "drink" the water to survive.
The main thing is that FRESHWATER fish do NOT need salt in their tank!!