The problem with putting salt in with fish that originate in salt free environments is that their kidney is put under greater stress then it would otherwise be. The kidney maintains the electolyte balance between the fishes internal fluids and the environment. The extra loading at low salt concentrations is rarely enough to kill outright, but is enough to make the kidney "burn out" faster then it normally would. Thus the fish dies younger. The higher the concentration, the sooner that happens.
A non salt tolerant fish can thus appear to be unaffected by the salt, but it is under stress nevertheless.
It is certianly true that many aquarium fish from line bred stock have a greater tolerance for sub-optimal conditions. The conditions kill off those that are less tolerant, so it is only the more tolerant ones that get to breed. That said, they may well still die young, it is just that they lived long enough to reproduce, but ultimately a fully tolerant race would evolve. QUOTE FROM LATERAL LINE.
This is why Plecos can not be kept in salt water, the only small botton dwelling brakish water fish I can think of is Bumblebee Gobies.
I know most can't but would like to know if there are any that could live happily in my slightly brackish tank. PS small species no bigger than 3 inches. 
