If you Google "goldfish grow as big as their environment" (there's your koi !) you get a bunch of references, including this in a box:

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.
Goldfish: Myths Debunked | Details | Articles | TFH MagazineŽ

So, if you accept their answer, the word you're seeking is "indeterminate."