What's the problem with your wellwater? In my experience...(my wellwater comes out of the tap at 7.9 ph, from a calcium/magnesium aquafir) if it's drinkable, it's fine for cannabis if ph is adjusted to within range. I use phDown (phosphoric acid) to lower the ph, but getting overly anal about water purity get's unnecessarily expensive if in soil. (do you worry about the purity of the dirt in your potting soil...? :wtf: )

Rainwater is usually acidic, and has whatever contaminates it has picked-up as it travels twords your region including industrial wastes, heavy metals, toxins and such.

The major problems I have with R/O, is the initial cost, the yearly filters, and the need to purchase additional nutrients to replace what's been removed. One gallon of phDown lasts me about a year and a half to two years. Much cheaper to adjust ph than to sterilize the water and have to replace what you've removed.

Avoid snake-oil remedies.