Howdy RagWeedDWC, I would not recommend using rain water in a DWC grow, even though the rain water absorbs oxygen and nitrogen as it falls thru the air, it also picks up mold and algae spores, dust, smog, and everything else that might be floating around in the air. In a soil grow, there are many beneficial microbes and bugs that will moderate the hazards in the rain water, where as in a hydroponic system, it is a breeding ground for all of these detrimental things. I think that one receives a greater benefit by using tap water, keeping it well aerated, and cool.