A betta fish can live in tap water, but the tap water must be conditioned correctly prior to housing the fish. Failure to do so may result in premature death.
Water conditioners are important to use for all types of fish, as they effectively remove harsh chemicals from tap water.
Stress Coat and SafeStart are popular options for conditioning a tank. Contrary to popular belief, it is best to keep a betta fish in a tank with a filter. This helps circulate the water and results in cleaner water for the fish.
Distilled water should not be used to fill up a betta tank. It's also important to keep the water temperature between 72 and 82 degrees Fahrenheit.