The GH is actually more crucial to fish than pH. Fortunately, the pH usually follows the GH. But the point is that if you deal with the GH, you will likely find the pH following suit. And I certainly agree that chemicals are not the way to do this.
The advice to select fish suited to your source water is one of the most important guides in this hobby. Aside from the normal day to day health of the fish (to which I'll return momentarily), when trouble like disease occurs and more frequent or larger water changes are called for, it makes life much easier to be able to use the tap water. I have in the past had issues requiring 75% water changes on alternate days, and this would have caused no end of problems if I had been unable to just turn on the tap to fill the tank.
However, hardening the water is much simpler than the reverse. I have zero GH/KH water with a pH of below 5 [they now add soda ash to raise the pH to 7, but this is temporary and GH/KH remains zero]. I have successfully kept tanks of rift lake cichlids and livebearers in the past by simply using a calcareous substrate (dolomite, aragonite, crushed coral, etc). You can also add a smaller amount of these to the filter chamber. I did this during the 1990's in two tanks to retain a pH of 6.4 (tap was then below 5). If you want livebearers, this is the way to go; adding chemicals is expensive and less reliable, and probably much harder on the fish. I would not mix soft water fish in this tank.
I agree that fish requiring moderately hard or harder water will not fare well in soft and acidic water. Fish need the "hard" minerals, and they obtain them by removing them from the water passing into the fish via osmosis through all the cells. Guppies may be something of an exception, and the deterioriation in the health of this fish in recent years may well be linked to all this. But platies, mollies, swordtails, etc. do need some mineral in the water.
The so-called adaptability of soft water fish to moderately hard water is not held by all in the ichthyological community. Some species show adaptability (not surprisingly, the natural range parameters of these species may be a clue), others do not. The hard minerals, especially calcium, in the water will be removed by the kidneys as the water passes into the fish by osmosis, but there is often the liklihood of this calcium increasing until it blocks the kidneys and the fish just dies. This was well documented in cardinal tetra, and there is no reason to assume it is not also occurring in other species. Only dissection after death will reveal it, as there are no external signs. The fish seems fine, but doesn't live the normal life expectancy. When one is able, providing water suited to the physiology of the species seems to me to be preferable if one wants healthy fish. The internal homeostasis of any fish will function better in water parameters for which the fish has evolved naturally.
Byron.
