I agree with the previous advice except I do have an issue with the notion that a fish can adapt to any water. If you've read some of my previous posts in other threads you'll know that I have a thing about pH. But my first comment, as mentioned by others, is don't mess with the pH. A pH of 8 is OK for livebearers, as previously mentioned. That said, I would find out why it went from 7.2 to 8. But now to my main point.
In unsuitable water and surroundings, fish become stressed and unhealthy. Although water conditions in an aquarium may be ideal, a fish that has developed (evolved) to survive in one environment will still suffer if placed in a completely different one. For instance, a fish that is by nature accustomed to a thickly planted stream with hiding places will feel very vulnerable in a bare tank that affords few or no hiding places. The fish does not "know" that it is safe and free from natural predators, and without the comfort of hiding places it will be continually stressed, eventually becoming diseased and may even die. The same holds for the water chemistry. The fact that many of our aquarium fish are today captive raised does not alter this; it is part of the biological makeup of the fish through millions of years of evolution, and you can't change the fish's natural "blueprint" by raising it in a tank.
The foregoing is a generality. Some adaptation is obviously possible, at least with some fish. But as aquarists I hope we are all concerned that we provide our fish with the best environment we can to ensure they are healthy and happy. As I can't know what a fish may be thinking or feeling (other than by observing the outward signs that can give a hint), I consider it my responsibility to provide the closest natural environment that I can, on the basis that the fish is probably going to be comfortable and healthy, and therefore happy, in that environment.
There is a biological process that occurs in all freshwater fish. All life needs water, and our cells (like those of a fish) are largely water. Ever wondered how a fish "drinks" water? It does so by allowing water to pass through its scale cells (marine fish are the opposite, water passes out, but that's another story). And in order to do this, it must change its internal pH to equal the pH of the water it lives in; the fish has no choice, it has to do this or it will literally die of dehydration. This is why a tank where the pH changes significantly and rapidly is so stressful to fish; it is continually having to adjust its internal pH to match the external. As others have noted, a fish can be acclimated to a slightly different pH if done very slowly and gradually. But each degree of the pH scale represents a ten-fold increase in the acidity or alkalinity of the water; for example, a change from pH 7 to pH 6 means that the water has increased its acidity by ten times, and a further change of that water to pH 5 means an increase in acidity of a hundred times.
Cardinal tetras live in very acidic water, and are found throughout much of the Rio ***** in Amazonia. A study carried out in 1992 over 720 square miles surrounding the confluence of the Rio ***** and Rio Demini found the pH of the water varied from 3.4 to 5.5 throughout the area [TFH, Jan 1993]. Expecting this fish, and other species from the same area, to "live' in a pH of 7.6 is not logical. Baensch & Rhiel [Aquarium Atlas, I] recommend an optimal pH of 5.8 with variances tolerated between 4.6 to 6.2, and when kept in hard water cardinals frequently perish from calcium blockages of the kidney tubuli. It is not to be wondered why so many aquarists buy cardinals and find them dead within weeks. I have for more than 15 years maintained my tanks at a pH in the low 6 range, and cardinal tetras have lived for 8 or more years (so far). I do not believe it is logical to think we can alter what evolution has put in the fish.