The lower Congo river is deep and complex, and there are a surprising number of hydrologic features that act as barriers separating populations of fish — this very nice video explains the diversity of species and the ongoing evolution of the fish in this environment.
They too briefly showed a blind depigmented cichlid that apparently lives in very deep troughs in the river — I wanted to see more about that. It’s probably out of the question to send divers down into that maelstrom, but cameras? Someday? Please?