Through Nashville, the Aquatic Critter isn't too far off the interstate depending on which direction you are coming from. They are the largest in Nashville. Another is Emerald Bay. Haven't been there in years. They had salt and fresh and a smaller store, but you could usually find something nice.
In Memphis, Kermit's Reef is one of the two I'd hit. I started my crack habit with Dave at Kermit's when I was in school in Oxford, Ms. He's usually got several things I want. He had a tank set up with 46 wrasses at one point. He's good with fishes.
The other store in Memphis is named Memfish. Richard's got a nice store, too. He had lots of things to look at.
South of Nashville (Lawrenceburg) is where Triton's Garden is located. They are a little off the beaten path, but worth the trip. They run it out of their house/garage, so you'd want to contact them in advance.
By the line of states, I figure Nashville and Memphis ought to cover your route:).