I live in uk in the countryside and built my own lean-to roof to catch rainwater . A clean sheet of pvc ,
I used it and on the 3rd water change I had a massive green water outbreak ,
That was the only thing I did different to my tank for over 2 years so it was obvious it was the rain water ,
Use it at your own risk , its probably hit or miss but like someone previously said ,
Would you drink rainwater ??
Where there's a will there's a way