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, <u>even when kept in good conditions</u>, will only grow to a proportional size of their tank.##

Unlike other aquarium fish, goldfish excrete a hormone called **[somatostain](https://en.wikipedia.org/wiki/Somatostatin)**, 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 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).