Ripley's Aquarium is located in the heart of Toronto right next to the C.N. tower and across from Rodgers stadium where the Blue Jay's play. This aquarium is located entirely indoors and takes about and hour or less to walk through.
While you won't find any big game fish here like whales, walruses ect., you will find very interesting and well put together attractions. The aquarium is set up so that you have to keep walking through in one direction. This is great because it keeps the flow of traffic moving.
Some notable attractions here would be the cleaner fish touch tank, the jelly fish exhibit and the shark reef which features a moving walk way. Tickets are around $33 for adults and $23 for children. While you can usually find discounts online it's still pretty steep price for such a small place.
Pro tip: Ripley's is opened until 11p.m. most evenings. Go after 7 p.m. to avoid the crowds and have a more enjoyable experience.