Rainwater?

My sister has this idea in her head that she's going to start using rainwater for her aquarium fish. Her reasoning is that it will be cleaner even than distilled water (Something about the plastic leaking chemicals into the water or something). Now, I don't know what kind of bacteria or whatever is in the air but is this a good idea? My sister's logic seems sound but there is always something, some factor, that gets overlooked.

I wouldn't use rain water for aquarium. There will always be pollution in the air and the rain brings it down. In most areas, the rainwater is not cleaner than tap water in my opinion.

There is no point using distilled water either, unless she is running a saltwater aquarium or if she has the equipment to make distilled water at home. With distilled water, there is no water buffer to speak of because all the minerals had been taken out. Any change in the water can crash the PH instantly. So if you use distilled water for aquarium, you must use a water buffer product such as Seachem Equilibrium to add minerals to the water before using for aquarium.

For freshwater aquariums, the easiest way is just to use tap water with a regular aquarium water conditioner such as SeaChem Prime to get rid of chlorine.