Here is a recent article I wrote for EzineArticles on the topic of salt baths, and NO table salt is good! Use sea or aquarium salt only.
"A salt bath may be the last thing you think could benefit your freshwater betta fish, but is actually a very effective, age-old cure that is one of the first steps you need to take to nurse your betta back to health.
How can salt help a freshwater fish, you ask? The bacteria and protozoa that attack your fish when its immune system is deficient are very low single cell forms of life. Common betta diseases are then transmitted to your fish by these simple life forms. They have almost no form of defense system, so even a gradual change in the level of salt in your aquarium will cause instant death to these primitive beings. They simply can not adapt quickly enough to the resultant change in their environment.
Why doesn't it harm my betta, you may ask? Fish are more complex beings, and have internal organs such as kidneys to help them deal with external environmental changes. They are much hardier than microscopic bacteria. Therefore a very minute change in the salt levels of their tanks causes them almost no discernible level of discomfort.
To learn what ratio of salt to fresh water to use, consult a comprehensive Betta Care Guide
for your particular fish. Usually, a ratio of one tablespoon per five gallons of aquarium water is fine for ongoing conditions, but this level needs to be significantly increased when treating betta diseases in a recovery tank or bucket.
Using salt to treat your sick betta is just one step to take to bring your fish back to full health. Once healthy, ongoing tank maintenance needs to be followed, and other tips and tricks can be used to ensure your betta lives as long and healthy a life as possible. Most bettas live about six months on average, but when proper preparation and care are employed, you can have a vibrant, lively betta for three to five years.
Want to double your betta's life? Need to know all about betta diseases? How about ongoing maintenance? Let a 20 year betta expert answer these questions and more in his best-selling Betta e-book Betta Fish Secrets