I've been collecting a lot of rain water, which is indeed very soft. However, it comes with some dirt that's washed from the roof.

One day while at Walmart I saw in the aisle with water filters that there was one kind of filter that was just yarn wrapped around a plastic cylinder, two for $5. I thought that was nice and cheap. So I looked up what the housing for those filters cost and they were only $15. So I bought one and some CPVC fittings to attach it to my pump and now I can filter dirty roof water and make clean rain water.