The Chattanooga aquarium is very nice. I have a season pass for my family. I first saw the Atlanta one 4 years back and was not blown away. It was nice, but not as incredible as I had heard. That of course was 4 years ago and I hear their reefs are progressing.
I also was formerly a Chicago native and went to the Shedd aquarium many times... I like the Chattanooga one better than it as well.
The Chattanooga aquarium does not have a true living reef, but the fish and small displays are great. They have a new Jelly section which is very cool. It offers a great deal more than you would expect from a smaller city. Well worth the visit!