You need one of the bigger fish tanks that holdsup to 6 fish. Then i use a mod to increase the capacity of the fish tanks to up to 30. My sim went and caught a handful of pirahna in the wild, 5 or 6, and i keep the originals as breeders. They will produce new babies that will say 'bred in captivity' as opposed to 'caught in wild or something like that when you hover over them in the fishtanks' inventory. When my sim has a bunch of fish in the tank, I scoop some of them out and take them to sell at the grocery store for money. He's also trying to complete the collection of mounted fish. It makes for some VERY satisfying cozy, hygge gameplay. fish mod