Goldfish are technically a species of carp, known as the Carassius auratus. Most carp grow based on their environment (or how much space they have), as well as the resources that are available to them. Domesticated goldfish, that generally dine on fish flakes and live in a tiny bowl, don't get very large. However, when presented with open waters, as well as more plentiful food sources, they can grow to much greater lengths. With that in mind, goldfish in the wild are also carnivorous, which may contribute to their shocking growth. That's right — without those store-bought fish flakes, your pet goldfish is more than happy to be trolling the bottom of a lake, snacking on the eggs of native species.