I've seen tons of recommendations for salt Here's just one
"Salt. Yes, bettas are not marine fish, but they do love a bit of salt in their water and it also prevents parasites and fungus. Add 1 tablespoon of aquarium salt, or rock salt per 5 gal of water (not table salt!!!! They are bettas, not pretzels!!). Now a day, I actually put 1 teaspoon per 10 gal. What that means is that you can play around a bit with your salt amount, and I have found that anything between 1 teaspoon per 10 gal to 1 tablespoon per 5 gal works. "