I do not put too much emphasis on the "best" range of pH. And I can give you several reasons why. Firstly, identifying what that perfect range is is a difficult task. Let me give you an example. If you look up what the perfect pH range is for lemon tetras, Rundle says 6.5-7.2. OK, good, but further research finds Baensch that says 5.5-8.0. A very different result. Then, searching even further, Axelrod says 6.6-8.0. Which one is correct? Surely some very different testing method were used here, since one result has a small range of 0.7 pH units, while another has a huge range on 2.5 pH units. Not only that, but I am sure that each study was not performed on the same individuals. So, with these discrepancies/differences no one number or range should be considered sacrosanct.
Secondly, in general, fish are far more adaptable than we give them credit for. While the fish will be at its absolute best in water that matches its natural habitat, most fish can adapt and live long, very happy lives well outside their preferred range. This is where research becomes important, you have to know how sensitive your fish is. For example, discus have quite a reputation for being very sensitive, whereas the common goldfish are very adaptable. Plus, the scientific research further demonstrates this point. On average, a fish can change its internal pH several pH units an hour. This is not the "three days" that some people claim it takes for fish to adjust, just really very quickly. I should further point out that that same research actually demonstrates the importance of water hardness compared with pH. The fish changes its pH by exchanging minerals in its body with minerals in the water, which as fishkeepers we generally lump under the term hardness. If the fish is moved to water with a large change in hardness from where it was from, there is either more minerals in the fish's fluids or more minerals in the ambient water. Either way, the minerals will want to balance themselves as quickly as possible. In general, a change in mineral content (change in hardness) also does have a change in pH. I.e. in general high hardness water has a higher pH, and vice versa, though it is not always true by any means. When the change in minerals occurs, the fish does not have control over the minerals as they try to balance themselves, which means the fish does not have control over its pH for some time. This is the "pH shock", though it really is not caused by pH at all. It is caused by changes in hardness.
If you look around the forum you can find many examples of fish that have lived very healthy lives outside of their preferred pH range. I can even give you a personal example, my water has a pH of around 8.2 to 8.4. I keep tiger barbs, bronze cory catfish, and lemon tetras. Note that 8.2 is outside of all three pH ranges I gave above. In fact, it is outside the preferred range of all three species I listed there, nevertheless, I have observed all of them spawning at different times. This forum is full of such examples. If the fish are spawning, they are living exceptionally happy lives, and yet are outside the pH range.
Finally, to adjust your tanks pH, you normally have to add chemicals in some form or another. pH up, or pH down, or something to that effect. Many members are loathe to add chemicals to their tank, and I agree. These chemicals can lead to pH crashes, many are phosphate based which can lead to explosions of algae. If you use CO2 to lower the pH, you have to check on the CO2 constantly. Etc., etc. etc. Considering that most fish can adapt like I showed in my second point, in the vast majority of cases I think it is far better to just let the fish adapt to your tap water. Then, you don't have to worry about trying to modify your tap water, the fish will become acclimated to the tap water and there won't be an extra hassle whatsoever.
So, for all those reasons, I really don't place too much emphasis at all on pH boundaries.
