Natives. I don't know anything about LFS in the region; my uncle in the Shoals keeps a marine nano tank and is always complaining about the paucity of good local fish stores.
The Tennessee is the most ichthyologically diverse system on the continent. The mainstem is pretty well screwed, but you're close to some of the most diverse tributaries, including the Duck, Buffalo, and Elk. You're also very close to the headwaters of the Tombigbee, Black Warrior, and Hatchie, all of which are very diverse with high levels of endemism. The region also has several endemic spring and cave species.
You're into fossils, right? Do you know about the Coon Creek formation, just to the west of Pickwick? Lots of great Cretaceous fossils there; it's famous for its mososaurs. The whole area to the east and north is Mississippian limestone, chock full of rugose corals, brachiopods, and such.