any fish with scales can tolerate it at 1 tbsp per 5 gallons, brackish fish even more. now here comes the debate. scaleless fish. they are very sensitive to it but my self and other like playfulalliecat and les gerber use a half dose 1tsp per 5 gallons and have had no problems at all. Snails can not handle salt at all. as far as what salt does, reduces stress, adds electrolytes, improves gill function, aids in disease treatment (speeds the process up) or in the case of brackish fish a salt bath can be used that kills parasites and really aids bacterial infections.
hth
richard