I know most places sell goldfish for bowls and tanks. But really, they are not meant to be kept that way. They can easily grow to 12 inches long if / when properly cared for.

Because they are so plentiful and easy to breed, they are an inexpensive way for folks to start with fish, especially to start youngsters with fish.

http://www.firsttankguide.net/goldfish.php

http://ezinearticles.com/?Goldfish-C...ance&id=933257

http://www.americanaquariumproducts....h_disease.html