Not all specie of fish can tolerate any amount of salt-you should always research that specie first before using any salt and/or medication...especially with corydoras an some tetra and plecos......
Also, lots of aquatic plants can't tolerate any amount of aquarium salt-some can like anubias, ferns and several others, however, plants can tolerate Epsom salt and is often used as a plant fert.
For Betta splendens-they can tolerate short term use of sodium chloride (aquarium salt) in dosages of 1-3tsp/gal...I like to limit treatment to 10 days-up to 14 days on rare occasions.
I use aquarium salt (sodium chloride) for external problems-limit to 10 days
I use Epsom salt (magnesium sulfate) for internal problems-can be used long term
However, if I need to treat both an internal and external problem I will use Epsom salt 1-3tsp/gal.
Both Epsom an Aquarium salt have antibacterial/fungal properties