The Petropolis Aquarium is an asylum for the criminally insane. It has only been seen in The Dog Who Cried Fish, where the crazy Caped Cod was admitted to the aquarium after his maniacal plans to seize control of Petropolis were foiled by Kitty and Dudley.
- As seen in the image of the aquarium, on the building's rooftop is a fishbowl with a castle. This could be a reference to Cosmo and Wanda's fishbowl home in another Butch Hartman animated show, The Fairly OddParents.
- The aquarium seems to take any criminals who are underwater beings.