Fish like some degree of electrolytes.
You still haven't said exactly which ones they are and why the fish want them. Until you can I think this is a null argument.
Some are intolerant, but even those like some amount
No. See, the word "intolerant" means they cannot tolerate it. In other words, some fish die with an extremely low amount of salt in the water. Previousl,y Bignose has found studies showing some species of cory die in extremely low amounts. These fish cannot be considered to be "liking the salt in small amounts" if they are dying from it.
because it encourages slime coat production and thus protects the fish from bacteria. Actually thats the main benefit of salt, It encourages the slime coat to build.
The main way that most (if not, all) slime coat additives work is to irritate the skin of the fish into producing more slime coat. Where is there any evidence that salt helps the slime coat rather than irritating the fish into producing more?
It's not oldschool, it's extremely current. I'd always reccomend some salt.
The use of aquarium salt came about from two outdated beliefs in early fishkeeping:
1) As "old water" was considered good, to prevent problems from nitrates and nitrites in the water, salt was added. This dulled the effects of poisoning the fish with the aforementioned nitrogenous waste compounds;
2) As FW fish maintain an internal salt level higher than the water around them, it was considered less stress on them to have a higher level of salts in the water. Unfortunately these earlier fishkeepers forgot that FW fish have evolved through millions of years to osmoregulate in the waters effectively devoid of salts, and by adding more it can cause problems with the osmoregulatory functions of the fish.
Nowadays people realise that water changes are far better than old water, and people try and keep the fish in water as close as possible to that in which the fish occur naturally.
Ground contains salts, rivers ruun over the ground. Any river in the world that you go to, will have some salts in it.
Indeed. But your tap water already has more salts in it than most of the South American waters. Freshwater rivers in SA (and most of Asia) contain less than 1ppm sodium chloride; an amount so small that even scientists don't bother to recount it any more accurately than "<1ppm" in degree level ichthyology texts.
Now then, since you claim the salt is so good for the fish, can you find any scientific evidence to support your statements that all fish benefit from salt in the water, even those that it kills in small quantities?