From Japanese Mythology...

The carp is also a symbol of youth, bravery & strength in Japan and Korea, and in Korea especially, it is seen as a symbol of wealth.
According to legend, a monstrous carp lives in Lake Biwa, the largest lake in Japan. Any who drown in the lake are devoured by this fish.

Because of its association with youth, the carp is a popular decoration during the Festival of Boys.