When it comes to water conditions, too much credence is often given to clichés of region as opposed to specific habitats. This includes the notion that African = high pH, hard water and South America = low pH, soft water. Some understand the mistake in this and know that: a) not all African aquatic habitat is highly mineralized, high pH and b) not all SA habitat is low pH and soft water. However, some who understand this still conceptualize SA water conditions as divided along geographic/regional lines that inadequately account for the variations in habitat within the same region.
What's well known to science and not always to hobbyists was proposed by British naturalist Alfred Russel Wallace in the 19th century and still holds true today, with some refinement of the details: River water types in SA, though not SA alone, generally fall within the 3 basic types below:
Black water-- This is the soft, low pH, tannin stained water some imagine generally typifies SA habitat and species.
Whitewater-- In order to make things easier on myself while providing a good description, here's a quote from the link to the left:
Clearwater-- Neither tannin stained, nor muddy. Often between the two other types in water parameters, but some clearwater rivers are fairly mineralized/alkaline/higher pH-- or some portions of them are, or some portions of them are during certain times of the year.
The above is very well known in science literature and can also be found in hobbyist literature, such as that authored by Heiko Bleher. It is fundamental to understanding SA fish species and their native habitat and is more important to know and account for with some species than others. Most SA cichlid species adapt reasonably well to non-native water conditions-- within reason and within a certain range-- with the following general exceptions:
Temperature: Even the normal variation of wild-native temperatures stress some species. For example, surprising to many would be to know that some discus see temperatures in the low-mid 70s in nature, those in such habitats live with this but tend to be stressed when they see such seasonal conditions. Others, like many gymnogeophagus, are well adapted to fairly wide temperatures swings and even
need the seasonal variation to maintain good health. The bottom line is temperature is important.
Breeding-- Some species that can live just fine and without issue in higher or lower than native pH or mineralization, may nonetheless be less fecund, or even have difficulty reproducing at all. Here's an
example:
Sensitive species-- In varying degrees, some are simply more adaptable to dissolved solids/pH/hardness outside of native conditions than others. A few years ago, through an unintended mistake in one tank, I discovered red head tapajos geos are far more high pH/high alkalinity tolerant than Guianacara (sold to me as G. sphenozona). I 'mysteriously' lost a couple of guianacara before discovering the problem was pH was way higher than I thought. However, the red head geos didn't skip a beat.
The same river sometimes crosses more than one geologic or environmental zone and its water conditions may differ along its length or according to season. For example, some eastern slope rivers originating in the Andes will be harder and higher pH in higher elevations.
Here is another reference, with details slightly different than the one above. There's a good volume of such literature out there, but note that it can vary in the details somewhat, often according to when it was published or when the references it uses were published. This is because greater access in the past couple of decades has resulted in an earlier picture of the waters east of the Andes region being adjusted somewhat. However the basics are the same. (Parenthetical note in the quote below is mine and added for clarification.)
https://www.researchgate.net/public...nce_for_tambaqui_Colossoma_macropomum_culture
So-- if you want to be accurate: SA water habitat is not simply divided between eastern and western slope of the Andes, nor even the Amazon basin vs other regions-- in fact, geologically speaking, it's not that simple either. There are actually
several distinct geologic regions east of the Andes or in what we may think of as the Amazon region. A more accurate picture is black water vs clear water vs whitewater, though even at that there can be variation or overlap in pH, hardness, nutrients, and specific mineral profile,
including at times variation according to the particular stretch of the river or time of the year. Naturally, something similar can be said of lakes or reservoirs in the region
duanes
agreed in that past discussion, that there's a LOT more at play than just pH values, when so called soft water species, don't do well in higher pH values. IMO one shouldn't blame pH values, for sloppy husbandry practices, which in reality the latter is most likely the actual stress trigger that caused the fish to become ill, with HITH, or whatever. Far too often pH is used as a cop out. 
All my indoor tanks get at least that much water changed weekly, and none of them are stocked to the point where fish suffer if the power goes out and the filters stop. Filtration is a luxury, not a necessity.