08-18-2011, 05:00 PM
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As the one who has written almost all of the freshwater fish profiles, I may be able to help a bit.
I use the pH and hardness "numbers" that are the consensus of acknowledged reliable sources, many of them biologists. It is quite amazing how very few times I actually find a varying number, which of course gives weight to the reliability of the sources.
Some fish have considerable adaptability when it comes to hardness and pH--and one must remember the hardness is just as important to the fish's health and hardness and pH usually co-relate. Some fish do not. Many of the "adaptable" species will manage but show some form of obvious dislike, and that is mentioned where I have scientific evidence of the fact.
A difference of .2 or .3 is not likely to make much difference. But a number has to be given or else we have a wide-open field. Saying this particular fish requires a "slightly basic" water can be interpreted differently by different people, so a number is given as the "high" end. If I use 7.5, I would expect 7.6 or 7.7 to be non-problematic; but if I used 7.7 then others may assume 7.8 or 7.9 is OK. So there has to be a number ceiling to avoid confusion.
Last comment is that most of the fish in aquaria are soft water fish. There are simply more species in very soft acidic waters than there are in basic harder waters, simply because the waters in the tropical rainforests are by nature very soft and acidic, and that is where species evolve at astounding rates.
To illustrate, only this past week I came across a recent scientific study on the Black-winged hatchetfish (published in February of this year). Up to now, we have known this fish as Carnegiella marthae, which has a natural range throughout the floodplain of the Rio Negro basin, and there is a near-identical species in the Peruvian Amazon known as Carnegiella shereri. I happen to have both. The recent study examined fish collected throughout the Negro floodplain, with the unexpected result that there are now three distinct species of black-winged hatchetfish. Eventually they will get described and named. From the description is is possible I have two of them; I have long wondered at the difference in patterning [if interested, you can read more in our profile, I just revised it this week]. These three distinct species have evolved in isolation [though two of them were caught together in the same net] within the same river system. The dwarf pencilfish, Nannostomus marginatus is a similar example; the new species N. mortenthaleri and N. rubrocaudatus were initially thought to be simply colour variations, but are now recognized as distinct species that have evolved independently from a common ancestor simply by being in different creeks.