I've never used epsom salt, but used aquarium salt for salt baths. you use 1 tsp/gallon of water. leave the fish in for 5 minutes then move them into a mixture of 2/3 tank water 1/3 the salt bath for 10 mins so they do not go through salt to fresh shock. then move them back into their fresh water.
If the fish goes unconscious immediately move them back into fresh water, if they stay conscious then 5 minutes. You can do this 5 times a day safely.
Salt baths help many ailments. It is the best treatment I have found for parasites, I have also used it successfully for fin rot, ulcers, and cotton (fungus).
It is not good for scaleless fish like pleco's, cory's, loaches, etc. It will burn them and will kill invertabrates. But for scaled fish it is always my first attempt at treatment. I much prefer the holistic approach when I can.
Betta's take well to salt bath's as they are very hardy to begin with and can handle brackish like conditions. In the case of brackish water only fish like puffers you use a strong salt bath of 2 tsp. or 3 tsp. salt per gallon