Aq is sodium chloride, which is used to cure open wounds. In addition, out is a de stressor. Considering its salt nature, it discourages bacterial and some protozoan and parasitic growth.aquarium salt is usually dosed at 1 tsp/gallon, with a te dose after each 100% water change, which should be done frequently if your fish is ill in order to keep bacteria, etc at bay. It can be done in much higher doses, up to 3tsp/gallon in serious situations, but only under recommendation or when experienced.
It should not be used for more than ten days.
Epsom (unscented) , like in humans, is a natural remedy for constipation. It is dosed, like Aq, at 1 tsp/gallon the first one our two days, then dosed higher. I can't remember what the dosing schedule is right now, but ofl shares the directions frequently in constipation/dropsy threads.
Also, aq's salt crystals are much larger than the epsom, which are very fine. Both need to be pre dissolved in tank water before added to the water.
I believe cory cats or catfish on general are sensitive to either both kind or just Aq.