Some time back while researching for an article on TDS, I came across this information which may be of interest. Source is:
Jensen, Niels (2009), “The Importance of Total Dissolved Solids in the Freshwater Aquarium,” as reposted on PlecoPlanet at
http://www.plecoplanet.com/forum/showthread.php?t=3480
TDS and Low ph fish
As I stated earlier, when we talk of softwater fish or low pH fish, what we really are saying is that they are low TDS fish. Even though many of these low TDS fish have adapted to harder, higher pH water, (and often relatively higher TDS levels), there is one fundamental problem, especially for South American dwarfs : the eggs cannot adapt well in calclium and magnesium rich water, and cannot hatch. My own experiences tell me that high TDS levels (in my case, mainly due to high levels of calcium and magnesium ions) are the culprits. This is where I used to make mistakes. I would use different methods for lowering pH and hardness but would take no consideration of TDS values. Peat can solve this problem.
I now use peat to successfully lower calcium and magnesium levels and thus pH (as well as KH and GH), as well as TDS. Many of the dwarfs I tried this with seemed to be much more content and this was reiterated by their ensuing fry (cacautoides, apistos, ramirezis, and other S.A. dwarfs.
If you match your own water with the fish's natural water chemistry, keep TDS levels low. My own experiences have taught me that the mineral content of the water, specifically magnesium and calcium, followed by carbonates and bicarbonates are the main reasons for the unsuccessful breeding of many South American dwarfs (they don't care for salts). Putting too much emphasis on pH does not solve the problem since one can say that pH is a kind of symptom of the mineral content of the water, not the actual cause. When you think of breeding dwarfs, think TDS.
In addition, it is my understanding, though I have no technical data or proof on this, that the blackwater rivers in South America are low in TDS levels, and basically sport some of the cleanest natural rivers in the world. According to
Mongabay, "blackwater rivers are very low in dissolved minerals and often have no measurable water hardness. The very acidic, almost sterile water, with a pH between 3.5-6, keeps parasite and bacterial populations to a minimum… often compared to "slightly contaminated distilled water… blackwater rivers are nutrient poor."
Whitewater rivers are richer in minerals compared to blackwater rivers, and often sport a slightly higher pH and are generally much more laden with minerals and nutrients. It is important to note that the rivers are not uniform and there can be drastic changes in water chemistry especially when comparing black-, white-, and blue-waters. "Because whitewater rivers are often fed by a large number of acidic tributaries, they are relatively soft in terms of water hardness due to their relatively low mineral content and have a slightly acidic to neutral pH (6.3-7.0)" (souce : mongabay).