When my fish-house was in full swing, I collected rainwater from its corrugated plastic roof covering into a garden butt. Filtered through Carbon as Andy has suggested above. This was mixed with a little tap water to get me the stats I needed for breeding Killifish (soft water). I was lucky. I have moved house now and the rainwater where I now live is, to say the least crap. It depends where you live and the amount of industrial and other contaminants contained in it. Try it, test it, take readings and see. Depending on what you are keeping you will have to add something back to it as I did, because mine was mighty soft and had a very low pH.
Regards
BigC