It may also arise from experiences with native fish such as catfish, gars, and bass. A general rule being the bigger and deeper the river, creek, or lake lake the larger the fish specimens that can be found. To be more specific I often fish for flat-head, and blue cats along the white lick creek a small fairly shallow creek system. If I caught a catfish there in the 15-16 inch range I would consider it large. I fish white river a larger system on occasion and my expectations for a large fish there would be over two feet. I could continue this into the very large rivers such as the Ohio or the Mississippi and the sizes would likewise increase. That would seem to indicate that the size of the body of water in which they are living is effecting their maximum size of the fish. Therefore, It would seem reasonable that the size of aquarium a fish is kept in would, to some extent, effect its maximum size. As experienced hobbyists we have learned that this is not the case in all species and that stunting the fish's growth by confinement also adversely effects their health, so we try to keep our fish in tanks that allow them to reach more natural adult sizes.
. After putting the goldfish into a larger tank the tail changed back to normal after about 3 months. 