The Koi Fish & the Samurai Legend.

According to Japanese legend, if a koi succeeded in climbing the falls at a point called Dragon Gate on the Yellow River, it would be transformed into a dragon. Based on that legend, it became a symbol of worldly aspiration and advancement.

More generally, the Japanese associate koi (also known as carp) with perseverance in adversity and strength of purpose. Because of its strength and determination to overcome obstacles, it stands for courage and the ability to attain high goals.

Koi as a Samurai Warrior

Koi Symbolism is also very prevalent in Japanese culture. It is said that a Koi swims up rivers and climbs waterfalls with bravery not unlike a Samurai warrior. As well, it is said that if a Koi is caught, it will lie still beneath the knife, facing its death bravely like a Samurai facing a sword.
Koi fish symbolism has gained strength over the centuries to encompass numerous positive qualities related to courage, overcoming adversity, the ability to attain the highest goals, and strong character.

The preceding are & were excerpts borrowed from Answers.com and Everything.com