Aquarium salt comes in the form of salt. Like sand pretty much. Just dissolve the right amount in the buckets during a water change. If its an emergency, add it directly to the tank, but slowly as a huge salinity change is bound to stress the fish.
also, 1tbsp to 5 gallons of water if you didn't know that. I dissolve it in a cup of warm water when I am treating the tank for the first time, then I can add it slowly over the course of a few hours, or half at night and half when I wake up.
Epsom salt is used to treat bloating, also as a source of magnesium.
Regular salt (sodium hydrochloride) has multi-fold uses. One it has anti-parasitic properties. Two the amount of TDS in the water will help with the fishes process of osmo-regulation.