Thanks Bas. Anecdotal evidence points to the same, slightly higher conductivity in fish tanks compared to nature is beneficial to those same fish.
I am not saying it applies to every black water fish species but I remember reading a blog on keeping kuhli loaches some long time ago. After keeping the loaches in very acidic low conductivity black water tank for years the poster found out they'd develop spinal deformities and recommended they be kept in slightly harder water.
Fish tanks aren't the same as nature in any way. I've personally kept my kuhli loaches in my hard water for quite a few years now, I think I bought them 6 years ago, and no issues at all so far. I lost one during a tank break down and another by taking out a planted basket without seeing there was a kuhli loach in it. I found it dried out afterwards.
A few weeks back I also found a baby kuhli loach in another plant basket while I was replanting. This is happening in water with TDS from tap around 300ppm but besides being rich in calcium and magnesium, my tap water is rather clean and low in other pollutants even though farming is a big thing in the area.