On Killietalk they compared the effectiveness of RO water, water run through an ion exchange system or distilled water (increasing expensive as one goes down the list) as opposed to using rain water to cut their tap water (if relatively unpolluted) and to enable rainforest fishes (some killies, Apistogramma, several other South American cichlids, many tetras, Rasbora, a rare few livebearers and other "problem fishes") to spawn. Why, they weren't exactly sure, but the rainwater assisted fish seemed more responsive than those whose water was cut or remade by the industrial methods of demineralization. Certain absorbed trace elements and better oxygenated water were suggested.

By the way, if one were to add fish to straight RO, distilled, or rain water (from recently cleaned catching tanks), the fish would suffocate if the chemical shock hadn't already killed them! Really "pure" water can hold no oxygen. It is always necessary to add some regular water or a mineral mix to it. And usually to wait a couple of days for free O2 to be absorbed.

I know of a couple of local ladies who decided that they were going to become the discus kings (queens?) of the Southern 'burgs here. They put a lot of their discus and other cichlids in the straight RO water and immediately noticed something terribly wrong with their struggling fish. That little fiasco cost them 100s, if not 1000s of dollars. Sadder yet was the demise of some neat fish and favorite pets before they could get many of them out of that water and into "old" aquariums.

There are a lot of ways to chemically shock fishes. Some of them I'm sure we haven't even figured out. That is why one, as a general rule, shouldn't add more than 50% "new" water to their tanks.

It's interesting what you mention in terms of the taste of town water. Ever since they started protecting us from the pathogens in the water, it hasn't tasted as good as it used to. I understand that "the good old days" are partly the result of very fallible memories. Still, I never tasted more wonderful water than that drawn from a deep well on my grand-dad's Wisconsin farm. Supposedly it flowed all the way from Lake Superior. And it was cold, seemingly pure and had a clean, fresh taste our tap or bottled water will never match. :)