As I was the "other member" mentioned, I'll just coment here for the record so everyone knows my thinking on this issue.
Fish have evolved to suit their specific habitat. While evolution is still occurring in nature, the changes generally occur over long periods of time in the case of vertebrates. The biological metabolism of a fish is geared for its environment, and while it is true to say that most fish can probably adapt to a different environment, such changes are complex and varied for each species. It is not something that can be done over a couple of hours such as introducing a new fish to one's aquarium. We are talking about altering the basic physiological metabolism that makes the fish a fish in that particular environment.
Second to nourishment (obtaining food), reproduction is the most basic instinct for all animal life. It is a known fact that many fish will not spawn in water that varies significantly from their natural habitat. If the fish is so affected by the water parameters or the environment that it cannot follow this basic instinct, it is illogical to say the different water parameters or environment is not of major significance. Also, pH and hardness can affect the motility of a fish's sperm, the hatch ratio of their eggs, and the survival of the fry.
The metabolism of a fish is complex. The pH affects how hard the fish must work (use energy) to maintain the physiological equilibrium--keeping the pH of its blood steady, feeding its tissues, and keeping its immune system functioning. Hardness can have other effects; cardinal tetras are known to develop calcium blockage of the kidney tubes when maintained in hard water. And it is logical that other fish from soft water would face similar issues. Such fish may "exist" for a period of time, perhaps a few years, with no apparent problems; but who among us can see inside the fish's body and metabolism? Cardinal tetras will llive for more than ten years in an aquarium with the proper water parameters; in less than optimum water this seldom occurs.
Which brings me to the common ram, Mikrogeophagus ramirezi, mentioned. Although most of the fish now available in stores are commercially raised in the far east or (perhaps) Florida, the fish rarely live long in aquaria; and I believe it is solely due to the water parameters. In spite of captive breeding, this fish has obviously retained its need for very soft, acidic and warm water. Several members here have written of failures at keeping this fish, and some of them recognize the water chemistry to be the probable reason.
In nature there are fish that show considerable adaptation. Pristella maxillaris is found naturally in soft water and in brackish water. Mikrogeophagus altispinosus, the Bolivian Ram, is found in acidic waters and in basic (pH 7.6) waters in South America. These are fish that somehow have the ability to live and spawn in varying water conditions. Dr. Chris Andrews, Director of the Steinhart Aquarium in SF, said that the reason for this is still unknown. He suggests it is always better to match your fish to your water chemistry than the reverse.