The Koi is a cold water fish, who when in the wild gain strength by swimming against the current.

Legend tells that many years ago, a huge school of Koi swan up the Yellow River.  The thousands of fish were doing well until they reached a waterfall.  Some Koi immediately turned back, opting for the easier route with the flow of the river.

A small but determined number of the school continued with their journey, leaping to reach the top of the waterfall.  Their struggle drew the attention of the local demons who were amused at the continued efforts of the Koi.

The demons even increased the height of the waterfall. But the Koi refused to give up.  Legend says that they continued their struggle for one hundred years, until at last one Koi reached the top of the falls.

God smiled down in approval and transformed the Koi into a Dragon, golden and shining.   Now whenever another Koi has the strength and courage to leap up the falls, they too are transformed into a ‘Heavenly Dragon’.

The waterfall has since become known as the Dragon’s Gate, and because of their strength, courage and endurance, Koi have become symbolic of overcoming adversity and fulfilling one’s destiny. (Source)