Just reading a lot on the forum .Saw a lot of post with the same problem and also people like Sakura,ANHEL and some other helpful people helped.
For the Aquarium salt and Epsom salt i learned from this forum
Aquarium salt (Sodium chloride) for injuries,wounds,external parasite,fin damage/rot(not plant tolerant)
Epsom salt (Magnesium sulfate) for buoyancy issues,swim bladder problems,constipation(plant tolerant)
Both have a antibacterial/fungal effect
Both you can use in doses of 1tsp/gall up to 3tsp/gall depending on what/why you treating . Also make 100% water changes daily with most treatment and use no longer then 10 days, in some cases up to 14 days.
It better to keep them in the hospital tank and maintain a water temp of 76-78*
Always pre - mix salt to make correct daily dosage and water changes easier.
But don't use aq salt long term though. Its doesn't prevent disease or parasites and long term can cause resistant pathogens/parasites issues and kidney damage...and long term non-therapeutic dosage can limit it ability to work for treatment when needed.
With external parasites though i saw some bettas died and some recovered, but also people were using medications . Some people used medications before salt , others after salt treatment.