True Parrot Hoplarchus psittacus and Geo winemilleri are from acidic black water of SA. Their native water has very low pH and almost zero hardness. These fish have not been domesticated very long. Even long-time domesticated Geos such as Jurupari and surinamensis are suceptible to HITH. I have kept both of these species and although they thrived in my 7.5 pH tap water and grew to huge size, they were disfigured by HITH.. Their tankmates from hard water CA and Africa were all healthy so I know it is not water quality or nitrate problems. Your Vieja melanuras should be fine as it is from hard water CA.