I feel like someone (with more fish experience than me) should put up a "salt" sticky lol. Recently lots of questions on this forum re: epsom salt, AQ salt, table salt, sea salt, etc...
If someone could post dosages and when to use information that would likely be helpful.
In general though:
1. Table salt: is NaCl- it also has potassium iodide added to prevent goiter in humans, and most contain some calcium for anti-caking. Someone correct me if i'm wrong but I cannot think of ANY indication for its use in freshwater fish, and would think it could likely be quite toxic given the additives.
2. Epsom salt is simply magnesium sulfate. This is known for use with muscle soreness in humans but is more commonly known for its laxative properties. Magnesium is what is in milk of mag, dulcolax, etc. The ones sold for muscle pain commonly have additives, only the pure ones should be taken orally, and should be considered for use with fish. I would assume indications in fish would include constipation, and potentially SBD secondary to constipation, also with bloating as a symptom of other disease, but I'll defer to the fish gurus on here to give a list of uses and dosing for bettas.
3. Aquarium salt, or evaporated sea water- basically NaCl- similar to table salt but without the additives. Seems to be recommended in small doses for a variety of betta diseases, so I will definitely let someone else tackle the uses and dosing (i've seen several different dosing suggestions).