Salt Bath's are a very useful, holistic method to cure illness in sick fish. It works well on parasites, ulcers, fin rot and some other fungal and bacterial infections. There are times when stronger meds are needed such as anti fungals and antibiotics. Salt baths are also not good for all fish. It will burn the skin of scaleless fish, but our betta's have scales and are rather salt tolerant.
To administer a salt bath
Mix 1 teaspoon aquarium salt with 1 gallon of treated water (not tank water), make sure all of the salt is dissolved. You can then poor the gallon into smaller containers if you are treating multiple fish. The salt bath should be the same temperature as your tank, so your fish does not get shock form temperature difference as well as salt.
Scoop your fish into the salt bath and leave them in 5 minutes, unless the fish goes unconscious (rolls over on side and gill movement slows).
After the 5 minute period mix tank water into the bath to get 2/3 tank water, 1/3 bath. Leave them there for 10-15 minutes, ten return them to the tank.
You can do this safely 5 times a day for very sick fish. If you catch an illness early 1 treatment is often enough. Fin rot is a prime example here. at the first sign of rot, give the salt bath and add a small dose of bettafix to the tank as a booster and it is usually enough.
If disease has gone on a while the salt bath is not enough, but is always my starting point. It is far less stressful on fish than stronger medications. If a fish does not show improvement after 2 days worth of baths then it is time to look to stronger medication. If they look worse after the bath stronger meds are also needed. It is not a cure all but is a great starting and often ending point for treating your sick fish.
Do not give salt bath's to any scaleless fish such as catfish and loaches, as well as snails, snails really hate salt.