They do come from highly alkaline, low nitrate waters, but because argentea is a riverine species, it has the ability to adapt to fluctuating water parameters, when rains come, pH shifts, dry season also different.
That said, it is most important to maintain high water quality for these fish, so if trying to mess with pH would make water changes harder(less frequent), then using your normal tap water as is, is the best medicine.
That said, it is most important to maintain high water quality for these fish, so if trying to mess with pH would make water changes harder(less frequent), then using your normal tap water as is, is the best medicine.