Rain barrel water.
Is there a recommended treatment to preserve the water collected?
Denton County Texas
Rainwater if filtered before it enters the rain barrel does not require any treatment.
The only time treatment is necessary is if something organic rots in the rain barrel. If this occurs clean out the barrel and add enough bleach and water to form a 10% bleach solution. The choline will gas off. then you can disperse the water and have the barrel ready to collect rainwater.
I use window screen insect netting to filter my rainwater before it enters the rain barrel.
To learn more go to this web site and see the 'Making a Rain Barrel' publication https://dallas.tamu.edu/hot-topics/water/publications/