Accurately adjusting pH can only be achieved by adjusting the GH and KH. The pH will be relevant to these most of the time; exceptions are situations like mine where the GH/KH of our source water is basically zero, and the pH would naturally be very acidic, so they add soda ash to raise the pH to around 7. But this is not permanent, and in my tanks the pH is much lower, depending upon the individual tank. But left on its own, the pH will be relative to the GH/KH and the environmental factors like organics. The more organics, the lower (more acidic) the pH, subject to the buffering capability of the GH/KH. So while wood, dried leaves, and normal fish excrement would all work to acidify and lower the pH, this may be offset by the buffering of the GH and KH (latter in particular, but GH does factor in as that is thee dissolved mineral). I tend to let the pH do what it wants as it will be more stable long-term. The more we target the pH, the more complicated the water chemistry becomes, and changing one factor can have unexpected reactions.
The numbers on SF for GH are ranges that should in most cases allow the fish to manage. Obviously the only accurate guide is the habitat waters. These are remarkably stable permanently. For example, in the Rio Negro, the largest blackwater river in the world and the sixth largest tributary of the Rio Amazonas, the GH and KH are zero and the pH is in the range of low to mid-4. More than one study has noted pH of the high 3's. The environmental factors are so significant and stable, that the parameters simply do not fluctuate. You might think a pH of 3.9 in one area and a pH of 4.5 in another (these are actual reported test results) indicates fluctuation, but that is not the case, as these numbers tend to remain steady in those ecological areas. Even temperature variation throughout the year is a degree or two at most. So aiming for parameters in the aquarium that are as close as reasonably possible to the habitat of a species will obviously mean a better chance of healthy fish.
Which brings me to the matter of adaptability due to commercially-raising species. A study in the 1980's in Germany on cardinal tetras (Parachierodon axelrodi) determined that lifespan was directly related to the mineral content of the water (other factors being equal). The higher the GH, the shorter the lifespan. Upon dissection following death, calcium blockage of the kidneys was determined to be the cause of death. Clearly this species has a preference for very soft water, and providing that will allow for a healthier fish. How many generations of tank rearing will it take to alter the fish's physiology to the extent that it somehow manages to flush out calcium? This is how the kidneys work. I am skeptical that the physiology is going to change significantly if at all. Different species may show different levels of tolerance or adaptability. But forcing the fish into such a situation also has other risks, such as stress. And how is the metabolism going to be affected (weakened) if the fish is having to deal with something it was not designed to deal with? Another thing about commercially (tank) raised fish is their weakness in general; these species are usually much more susceptible to various pathogens than wild fish. This is probably not a coincidence, but a result of all this attempted tampering with nature. That does not always mean replicating the values of the Rio Negro, but my own view which is shared by the ichthyologists I have read or conversed with, is that the closer we get to the habitat the better. The late characin authority Dr. Jacques Gery wrote that cardinal tetras usually live for 2-3 or 4 years; but when provided with very soft water, they will live beyond 10 years. I've no data on the water that is being used for tank-raised cardinals in the EU, nor how long those fish can be expected to live.
