The short answer
Rainwater can be safe, and it’s naturally soft and mineral-free like RO — but it comes with two big caveats. First, it has almost no GH or KH, so like RO it has no pH buffer and must be remineralised or blended before use. Second, it can pick up pollutants — air-borne contaminants, roof chemicals, metals, bird droppings — depending on how you collect it. Collect it cleanly and treat it like a soft-water base, not a finished tank water.
Collecting it cleanly
The cleaner the collection, the safer the water. Collect straight from the sky into a food-grade container rather than off a roof where possible. If you do use a roof, avoid ones treated with moss killer, discard the first flush that rinses off accumulated grime, and store it covered. In heavily industrial or polluted areas, be extra cautious — rainwater there carries more.
Why it needs treating
Fresh rainwater is essentially soft water with near-zero minerals and buffer. Used neat, its pH is unstable and it starves plants, shrimp and snails of calcium and magnesium. Fix this exactly as you would with RO: remineralise to a target GH/KH, or blend with tap water to keep some buffer. Test the result with a liquid test kit first.
Is it worth it?
Rainwater is free and soft, which appeals if your tap water is very hard. But the pollution risk and the need to remineralise mean it’s fiddly — many keepers find RO more predictable and consistent. If your tap water already suits your fish, you probably don’t need either. See using RO water, softening water, and the water testing hub.