Would depend where you lived in the world. Rain can still be acidic, etc. etc, even if you are many hundreds of miles from a polution source.
You could always test your rain water.
it would be o.k if you are doing fresh water fish...... rain water is quite soft and with salt water fish you need a higher Ph(hard water 8.2) or so ...... and if you do use it ...... boil it to kill any bacteria..
Hi,
typical rainwater, from an unindustrialised clean area, has a pH of around 5.6. In more industrialised/polluted areas it can be even lower. This is considerably far away from your desired pH of about 8.3.
Regards,
Tom
As a chemist I can tell you NOT to use rainwater. Rainwater can be more contaminated with undesirable compounds than you tap water. For a reef tank only go with RO or DI.