Why Domestic Goldfish Can Grow to the Size of a Small Dog When Released Into the WildTaylor Shouldice of Animalogic explored why domestic goldfish become feral and can grow to the size of a small dog when released into the wild. There are a couple of reasons why goldfish get so big when they go feral. Releasing goldfish into the wild is considered Unauthorized Stocking and is also illegal in many places! They can outcompete native fish for resources and habitat. And we need to keep our invasive, not native fish in our tanks and not release them.