Is rain water collected from a roof really that dangerous for an outdoor fish pond if added in small quantities (less than 5% water volume) or is it probably okay?

uberbatman will be able to answer this

It is fine. I have a pond which gets more than 10% of it’s supply from my roof. Actually after a hard rain once they have even spawned.

@gooch so you have a “spawn pond” eh

@rag it’s a koi pond. But the rush of fresh water induced them to spawn.

@gooch http://en.wikipedia.org/wiki/Spawn_point

As long as the roof is free of poisonous contaminants, you should be fine.

Rain water is fine. Animals and birds drink it all of the time Too.