Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I use rain water in season, but never the first rain, or even mid season if it quits raining for a while then starts up again. I let the first few rains wash off the roof.
Rain water itself has no minerals, but can pick up CO2 from the air, and certainly anything else in the air, and from the surfaces it flows over.
It is fine to top off a tank with rain water, or do small water changes so the mineral levels do not drop suddenly, or too fast. If you need to do a large water change I would add minerals to the rain water so that it matches the tank water.