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Must be a less known river as the only red river I'm familiar with is out west that flows into the Mississippi R.
I don't have a clue lol. I know nothing of the river beyond that it cuts through the very northernmost part of Vietnam then is in southern China for the rest of it's length. Once we see pics and get an idea of what it is, we can go from there. I doubt they're HLG, but if I were forced to guess at gunpoint, I'd say that they were likely tropical or subtropical in their needs.