Which is probably why many pet stores and such recommend salting the water when you add new fish... the salt DOES kill off some of the parasites that could hitch a ride in with the new fish.
It certainly isn't NECESSARY though if you don't have any reason to suspect you've an infectious parasite in the tank.
I had white spot in my tank and salt at concentration of 1tsp/l (1tbsp/ gal) did nothing. Salt may help healing or constipation, but i doubt it is useful against parasites.
I have always heard that some fish, Mollies for example, like some salt in the tank.
Correct.Since this question is specific to a blackwater habitat- DO NOT salt the water.
A blackwater river or stream is typically very very low ion content. They are high in disolved organic compounds and low in inorganic ions.