The "Wild Reef" expansion was completed in 2003, so you must have visited literally within weeks or even days of the grand opening. The 5 million gallon Shedd Aquarium is my all-time favourite, and it is generally regarded as one of the best on the planet.
I think it had just opened at the time of my visit in April 2003. There was a long line of people and once I got in I found the aquarium to be chock-full of screaming children, so full that I didn't even go into the Wild Reef and instead went to see what I came for, the Pacific White-sided Dolphins and the Belugas. I need to get to Chicago again, and then in the middle of the week. US zoos seem chock-full on weekends.