Beware of inaccurate info on the app!
We had C3 Chicago passes—the City Pass app says that timed entry reservations are recommended but NOT required for the Shedd Aquarium. We went without reservations and found out that reservations are indeed required. We had to wait around for the next available ticket time. We ended up strolling around the banks of the lake nearby while we waited but would have appreciated having accurate information in advance as we had made plans already for some other things that afternoon and that threw the timing of everything off.