While goldfish can grow up to 18 inches in length and can weigh more than six pounds, most never attain such large size. Goldfish occur in a number of different varieties, and some grow larger than others. The most common goldfish in state fairs and pet shops is the “common goldfish.” Common goldfish frequently reach between four and seven inches in length.
Goldfish do not grow to the size of their tank, but poor tank water quality stunts their growth. As many goldfish are improperly housed in small bowls, the water quality is almost always poor causing the fish to fail to reach their potential size. Goldfish are best housed in tanks that are at least 10 gallons in size rather than in small bowls that only hold one or two gallons of water.
While there are over 100 different varieties of domestic goldfish, they all descend from a single species that hails from China. Some are bred for very small or very large size while others are bred to have fanciful fins or bulging eyes. Relatives of carp and koi, the natural history and adaptations of the original species, known to scientists as Carassius auratus, allowed the fish to thrive in captivity.