The opening line to this paper says it all "The southeastern United States is inhabited by the greatest diversity of aquatic plant and
animal species in North America, and the most diverse freshwater ichthyofauna outside the Tropics".
The author goes on to describe the forces that have led to this diversity include rising and falling sea levels and the pulsations of the recurring ice ages. It way nerdy but there are fabulous nuggets of original insights in this paper.
He focuses on the Pygmy Sunfishes of the genus Elassoma, but the conclusions apply to all Southeastern Fishes.
We North American fish keepers barely realized how good we have here, but isn't that always the way?