Arapaimas may be 3 meters long and weigh 200kg - and there are more species than we thought. New research on these fish in the rivers of the Amazon basin has identified specimens with "highly distinct" genetic markers. It was only in 2013 that new research confirmed long-reported (but dismissed) evidence of a second species. Now we have a third.
Even in fresh water, we don't know all the fishes. We don't even know all the big ones for sure!