aquarium salt and epsom (plain ,no additives added) have antibacterial/antifungal affect. Both can be used in the dosages tsp/gall - 3 tsp/gall. Always pre mix make sure it dissolved first(no salt crystals should before you use it for the tank).
Aquarium salt you can use for 10 days full course of the treatment. In some cases up to 14 days. Epsom salt is very mild on fish and can be sued for long time. And in some chronic problem indefinitely in low doses.
Aquarium salt is for external problem- external parisites, fin rot/damage, wounds etc
Epsom salt for internal problem like internal parisites (help to purge them out, not treat though); constipation( laxatives); swill bladder problems, pop eye etc..
What wrong with your fish?