I have 1500 gallon tank up at the barn that collects rain water from the barn roof for my garden in the summer. (My well water kills plants.) The water is gravity fed down to a couple of hose bibbs at the house. I've used the system for the deck plants and garden for at least 15 years.
I've never tried to run it thru an
RO unit although I have thought about it occasionally. The water as is works for my plants and animals, and I've used it from the beginning (last April) in my fresh water planted tanks. I don't use it on the salt tanks, I buy jugs of
RO/DI water at the
lfs.
If you want to try rain water, make sure your roof doesn't leach toxics. My roof is metal and I've heard that tile roofs work also. Personally, I wouldn't use water off a composition roof and I'd do a lot more investigating before trying water from a shake roof. You'll also need to plan for turning the system off and draining it in winter if it gets below freezing in your area.
