I currently use half Reverse Osmosis (RO) water, half tap water & dechlorinator for my aquariums (probably closer to 75/25) but it takes a lot of water to generate enough RO to do water changes every week. For each unit of RO I create, I throw away at least 2 units. I know that I could use it for other purposes (such as water plants etc) but I still have to pay for that water.
I live in the UK so there's no shortage of rainwater falling out of the sky. As I am removing the total dissolved solids and, I think, the bacteria, there should be no reason that I cannot generate RO water with rainwater that I can see.
Can I safely make RO water using rainwater as the input and is my reasoning above correct?