Some magical powers attributed to salt in freshwater fish/aquariums: protection, healing and stress.
Salt and "protection." First, "Salt can aid in the production of the slime layer," you'll be told. And so could many water-born toxins or irritants, in fact. You're already aware that the fishes' normal slime covering is produced by specialized cells scattered through the epidermis. To increase their mucus production, these cells could be stimulated in two ways. One way would be through the action of a hormone. Hormones stimulate secretory cells of all kinds. But no one is suggesting that salt contains a hormone or is imagining that salt is some chemical precursor of mucus. In the other way, these slime-secreting cells could be stimulated by an irritant. After all, many irritants and toxins trigger hormones., and salt in the water merely acts as an irritant. If you've ever inadvertently half-poisoned a fish, as I have, you know that one reaction of the fish to any stressful irritant is to increase mucus production. It is true that the increased flow of mucus can help slough off incipient parasites. To this extent, you could justify saying that salt in the water "protects the fish from parasites." I find this to be stretching a point. Salting the water to increase the mucus layer is like putting a drop of lemon juice in your dry eye to make it water.
I've recently read that ammonia acts to thin and break down the slime layer of marine fish. Certainly we all know its action as a surfactant when we add a capful of household ammonia to the dishpan. If this is true in saltwater, NH3 might have a similar effect on freshwater fish. But surely you'd act to reduce ammonia levels in the water, rather than to compensate for ammonia by adding salt.
Salt and healing. The second common magical power attributed to salt is that it "aids healing." In a dry atmosphere, a salt solution helps "draw" degraded cells and pus material from the abraded tissues of a wound, thus helping to clean it. A painful procedure from old shipboard days, when lacerations from a flogging were prevented from festering by applying dampened salt, still echoes in the phrase "to rub salt in someone's wounds." This bracing concept can't be extended to a water environment. When you inquire closely from an ol' "salter" how salting the water would aid healing, you'll soon understand that it's always based on an idea of "hyperosmolarity," that tendency of a more concentrated solution to "draw" a less concentrated solution that is at the heart of osmosis. You can't "draw" a fishes' wound; brine that was more concentrated than its own blood and tissues would quickly kill a freshwater fish.
A peripheral thought. Salinity levels that approach brackish water would repress Saprolegnia-type water molds, which can attack necrotic tissue. That's not the active consideration, though, when folks say some salt added to fresh water "aids" healing.
Salt and "stress." There is another wide-spread misconception, that some salt permanently in the water is "easing the stress" of osmotic pressure. This misconception is actively encouraged by packagers of "aquarium" salt. You'll hear this old tale repeated so often that, if it came to a vote, it might be voted "true." Most likely, this mis-application of "stress" comes from the idea of osmotic "pressure." Any pressure, such as social pressure, must result in some "stress" to the organism, such as social stress--— that's the thought, anyway--— and if osmotic pressure could be reduced, or even equalized, so that the surrounding water were at the same concentration as blood and tissues, then osmotic "stress" would be reduced. The fish would have to do less metabolic "work" to maintain osmolarity in its blood and tissue. I've even been told that the energy saved could then be applied to fortifying the immune system. These "logical" conclusions aren't based on the actual physiology of freshwater fishes.
But this is a mis-reading of the meaning of "pressure" in this case. Though peer pressure may result in stress, not all pressures result in stress. For example, atmospheric pressure doesn't result in stress. And neither does osmotic pressure. Fishes have evolved to adjust within certain limits to salt levels, ranging from virtually zero to water more saline than average seawater. No species of fish however will thrive at every level. Not all fishes are tolerant of change: the few that are warrant a special designation, "euryhaline." You know these things. But you will still hear lots of well-intentioned talk about salt and "stress," often from quite experienced aquarists. Listen skeptically, and I think you'll recognize this basic mis-connection between "pressure" and "stress."
Other opinions of magic powers of salt are just chit-chat. Take them with a grain of, um, whatever...
Or compare this good cautious article from the Aquascience Research Group, which may dissuade you from adding salt to the freshwater aquarium, if I haven't been able to convince you.
Salt sellers. Okay, now you're prepared to decipher claims made for an "aquarium" salt by its packager. Here, word-for-word, are salt claims off a box of "Aquarium" salt:
"An all natural* salt, made from evaporated sea water**...providing the essential electrolytes*** fish need to survive in an aquarium: calcium chloride, calcium sulfate, magnesium chloride, magnesium sulfate, potassium chloride and sodium chloride."
*"All natural." Generic reassurance.
**"Evaporated sea water" will contain many salts beside sodium chloride. Calcium and magnesium carbonates, not listed, would affect the pH buffering and would raise the pH in unbuffered acidic water. Additional buffering may make it more difficult to reduce pH if you're administering a medication that is only effective at a lower pH. The sulfates and chlorides listed provide no pH stability.
***"Electrolytes" are any dissolved salts. Of course you know that fish can't survive long in pure distilled water alone. It's in this sense that some minimal electrolytes are "essential."
"Helps improve gill function* to reduce stress**... Reduces electrolyte loss and promotes healthy gill function."
*"Gill function" in this case refers to osmoregulation, the regulated passage of ions back and forth across the gill membrane. Salt's chloride ion blocks the uptake of nitrite. It's inconceivable that "gill function" could be identified with the gills' certral role in respiration.
**"Reduce stress" refers to osmotic "stress"
"Can be used with most* aquarium remedies to improve recovery from disease."
*Though this claim reassures you that salt will not interfere with the effectiveness of medications, "most aquarium remedies" must exclude all those that are rendered less effective by higher pH.
"Protects fish against nitrite toxicity.*"
*Yes! Yes, at last! Indeed, minute concentrations of chloride ions do inhibit the uptake of nitrites through the gill membranes.
-Quoted from an article in "1999-2005 The Skeptical Aquarist".-
http://www.skepticalaquarist.com/docs/health/salt.shtml