pH should be below 7, in nature, the pH can get very low 5 or even lower at the start of the rainy season. The rain washes humic acid from the fallen vegetation into the, at that time, small streams/rivers. Later when the rivers fill, it is diluted and the pH slowly rises. Many characin species spawn early in the season however, so lower is better.
Calcium and Magnesium ions in the water have a large impact on hatching. If the water is "hard", even if the fish spawn, the eggs just do not hatch.
When I have bred Amazonian characins, I have used an RO unit to get the water down to where it needs to be.