Why would you want to use rain water anyways?
In my opinion, if you're just looking for distilled water, it's not worth risking to collect rain water for your aro.
There's too many factors that can affect the water you collect, such as where you collect and how you collect it.
Not to also mention rainwater is usually acidic (air pollution), and your fish would definitely not survive from it.
You don't accomplish much by using rain water other than introducing possible toxic chemicals into your tank.
