Well it can be done but you need to make sure its clean and free of paracites. Make sure you test the rain water first. Also a steady supply of rain water is needed, to have usable rain water for 6months of the year is no good because a chang from rain to tap after months of soft acid rain water is a very bad idea.
If youre already using tap water you would have to slowly adjust the % of rain/tap water to prevent a dramatic ph drop. Its probably more trouble than its worth. 100% rain water would probably need mineralizing also unless someone can tell me different?
If you get fresh rain all year round its doable but sitting rain water will become stagnant and harbour nasties over time.