The Shedd Aquarium, a part of Chicago's museum campus of internationally-renowned institutions, is a sight not to be missed. Though it draws a ton of tourists during all seasons, you see a significant number of locals with annual memberships coming to check in on their favorite flounder.
The viewing tanks are large, plentiful, and well-maintained. The Pacific Northwest habitat for the dolphin show is beautiful, and the ground floor exhibit on coral reefs is one of the best exhibits I've ever seen at an aquarium. I still think the National Aquarium in Baltimore, MD has this fish haven beat, but it's only by a fin.