table salt contains iodine which can kill your fish. Some people say you can use table salt without iodine, but it should not be used for more than 2 weeks max. There was a person here not long ago whose fish lost it's fins due to burning from the salt and all that was left was a large white patch and a red raw looking place where the fin used to be. She quit using salt, and he began to heal.
Bettas are fresh water fish, not salt water, so salt should only be used carefully as a treatment, not a preventative.