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There is a widely recited saying that sharks kept in fish tanks won't grow to their full size.

For example, author and motivational speaker Bob Harrison posted:

A shark in a fish tank will grow 8 inches, but in the ocean it will grow to 8 feet or more. The shark will never outgrow its environment and the same is true about you. Many times we're around small thinking people so we don't grow. Change your environment and watch your growth.

Will a shark that normally grows to 8 feeet (2.4m) only grow to 8 inches (20 cm) when raised in a fish tank?

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This article is about goldfish, not sharks, but it's the same urban legend:

Goldfish Only Grow to the Size of Their Enclosure

There is an element of truth to this, but it is not as innocent as it sounds and is related more to water quality than tank size. When properly cared for, goldfish will not stop growing. Most fishes are in fact what are known as indeterminate growers. This means that, unlike humans, they grow until they die. What really stunts a fish’s growth is poor water quality and improper care. In smaller aquariums or bowls, water quality is typically very poor. With little or no filtration and infrequent water changes, goldfish suffer. The stunting that results is not a good thing. Rather, it is a sign of ill health, and, frequently, stunted fish take on a deformed appearance and die at a young age. As already stated, some goldfish grow very large, so it is important to know what size your fish will attain before purchase and to make sure you can provide it with the proper care it requires.

Goldfish: Myths Debunked | TFH Magazine®

  • Is it really debunked? I mean, how would a fish grow to beyond the size of their enclosure without first dying for any number of reasons? Seems more like a tautology, since there would be no scenario where a goldfish (or shark) could outgrow the size of an enclosure, so saying it's "true" proves nothing about the why. – PoloHoleSet Sep 18 at 18:07
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    Water quality in a fish bowl is determined by what the fish excretes. So a big fish in a small bowl will have poor water quality just because of its size. But also the fish will not just 'stop growing' but become sick if its tank is too small. – DJClayworth Sep 19 at 15:12
  • They do not grow until they die. Koi (AKA carp/goldfish ) have been documented to be over 100 years old , yet they are in the same size range as mature ( 5 yr) Koi. Genetics , and water quality of course have a strong influence. – blacksmith37 Sep 22 at 20:10

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