Rain Water For Goldfish
Can I keep Goldfish in rain water directly collected in a bucket? (from sky)
In a word...No.
Rainwater is very pure, with no dissolved chemicals in it.
If you put any fish in rain water, the fish's chemical balance is upset as the chemicals in its body leach out and it will eventually die.
If you want to use rain water because you have no other water source that is not polluted, put about one heaped teaspoon of un-iodized salt per gallon in the water.
Another thing to be careful about is the vessel you are collecting the rainwater in.
If your bucket is galvanized, some zinc will leach into any water collected. Zinc is poisonous to fish as are most other non-ferrous metals. Read more on Goldfish Dangers...
The last thing to consider when collecting rainwater for fish is atmospheric pollution.
Most large cities in the World have pollution. Let it rain for at least 30 minutes before putting out a container.