Question
As I grew up, I was taught that a goldfish will only grow to a certain point dependent upon the size of their container and how many fish are in the container; all the fish within the same container would be limited by this rule.
Is it true that the growth of a goldfish is limited by its container size?
Answer
Toxotoes answer references an article on trout studies. There's no basis for equating a study on trout with that of goldfish. Moreover, the study referenced doesn't even support the researchers hypothesis.
It is true, goldfish, even when kept in good conditions, will only grow to a proportional size of their tank.
Unlike other aquarium fish, goldfish excrete a hormones such as aminobutyric acid (GABA) and somatostain, that acts as signal of how many other fish and what sized body of water they are in.
In the wild, this will regulate their size and the size of others and help keep them from running out of resources. While the GABA / somatostain regulation is an example of a "crowding effect", it shouldn't be confused with the stress based responses common to all fish in which epinephrine and norepinephrine increase due to poor living conditions (bad water and overcrowding).
Refs:
http://jeb.biologists.org/content/203/9/1477.full.pdf
http://thefishvet.com/2012/02/28/do-goldfish-grow-to-the-size-of-their-tank/
Answered By - virtualxtc