I remember seeing an article, about a lady who was breeding some kind of freshwater fish but with a low ph.. and the problem she was having was a really high water bill when she ran the ro unit.. so what she did was, she got a bunch of plastic drums and had them outside against her house lined up and all plumbed together. then when it would rain, the rain would come off her roof and into the gutter and into the drums... Then the water went threw a window into her basement, where she had more plastic drums.. once the water was down in the basement, she had a pump that would run the water threw a filter, and then a UV sterilizer.. and she had a bunch of water to put into her tanks..
me personally i don't think i would do this, but I'm sure during the spring and fall months (rainy season) it could bring does the water bill quite a bit...
andy