You can find the fish in the kitchen. You can feed them any cookie crumbs you find in the house. Fish are much more generous in giving Happy Points and Moolah for crumbs. Each fish will give 8 Happy Points and 8 Moolah when fed (while crumbs may get 1 Happy Point and 5 Moolah at Trash Can), but fish should only be fed once per round, otherwise the subsequent feeding will yield the same Happy Point and Moolah as Trash Can. The game starts with 1 fish in the tank. By feeding these fishes, they will eventually grow to 6.
In this game, there are fish in a tank in the sushi bar. They are different from the fish in the first game. In Exploration mode, you can interact with them by tapping their tank. Once you do, some fish will get scared, and swim somewhere else.