rain water would probably depend on whats in it in your area. Rain water is different depending on area. Some rain water is acidic. I personally would never use it. R/O, Distilled, treated tap water, spring water, are typical waters used in the frog hobby. Most people use R/O or distilled for their misting because it doesn't leave hard water stains on the glass. For frogs that need a soaking dish, dechlorinated water is typical. I know there are people who use rain water for things but like i said, you'd probably want to get it tested before using to see whats in it.