Not a Goldfish
You might be asking yourself, aren't koi fishes just large gold fishes that are all full of themselves, and have somekind of splotchy skin disease? No, you'd be wrong. Koi are actually a type of carp, and when they revert back to their undomesticated forms when released into the wild, become evil and instatiable killing machines. Laying unholy waste to entire undersea environments, brutally devoiring all sea creatures who oppose them, and in a final act of defience destroying the very habitability of lakes and rivers for all those except themselves and their loyal vassel fish.
The koi are a dignified and intelligent form of life that were once worshipped by ignorant early humanoids as an underwater society far in advance of their own. Our ancestor's quiet contemplation of the war like wild koi invented the code of "samurai" which means cult of the fish people in Japanese. In the centuries that followed, the koi evolved into a lover of art, beauty, and the Buddhist study of meditation. And in doing so tamed our violent human hearts, making things like farming, steam engines, and cell phones possible.
For those who appreciate the koi and understand their long and frightening history, there's no greater joy to be found than in throwing a handful of food in the middle of them and watching them violently fight each other for sustenance, before settling back into their zen like fish trance. Completely at peace on the surface, with unbriddled animal rage just below the surface. That is the beauty of the koi.