Agree
There are of course, similar habitats across the globe, so mixing certain species may not always be problematic.
The African rift lakes have similar water parameters to much of Central America's, high pH, mineral rich environment.
So from a purely chemical perspective, no biggie.
The problems "can" become evident when trying to combine soft water species that have evolved less immunity to the certain bacteria found in hard water habitats.
The bacteria that easily causes HITH more often in Amazonian fish, because they have not needed to develop resistance in those soft water, high tannin conditions that prevent its proliferation in the Amazonian natural habitat.
So combining severums, Amazonian Geophagines, and others from water like this.
With Herichthys or Vieja, from very different water like the data below, can be problematic, on a microbiological level, especially in combination with unnatural high nitrate levels found in most aquaria.
But biotope awareness might even come into play for fish from the same river.
Many Geophagines come from fast flowing rheophillic conditions, whereas severum, and angels prefer oxbow low flow environments, and combining them in one or the other can cause stress.
This to me, is why simple awareness of biotopes is key to an optimumally healthy tank.
