Steven can/will respond...but no, this is not a correct statement. It depends what you mean by "most fishes." Fish that need moderately hard water in order to function (all livebearers, many of the rainbowfishes, rift lake species, and some others) cannot manage in softer water. In very general terms, a GH above 10 dGH (180 ppm) and a basic pH (above 7.0) are minimum for such fish.
On the opposite end, very soft water fish live in natural waters having a GH of zero, and a very acidic pH (in the 4's and 5's depending upon the watercourse) and these species must have comparable water parameters in order for their physiology to function properly. Fish like most of the characins in South America, and many of the cyprinids in South and SE Asia are in this group.
There are species that live "in between" these two extremes, if we may so call them. There is no "one size fits all" in freshwater fish requirements when it comes to water parameters. Note, I emphasized freshwater; brackish water species and marine (ocean) species are a very different "kettle of fish." Each speci8es of freshwater fish on this planet has evolved to function within a quite narrow environmental range, which includes not only water parameters but other factors. As soon as the fish is forced out of this specific range, its life becomes more difficult. The aquatic environment is the most demanding of all habitats for living creatures.
