koi

carp

In addition to being an important symbol, the carp is commonly found in moats, ponds, and canals in Japan. A carp banner is flown on Children's Day (formerly Boy's Day). Carp, or goldfish, are one of the Eight Treasures of Buddhism. The carp is a symbol of perseverance because of a Chinese folk tale involving carp swimming up rapids. The motif of a carp with cresting waves was especially popular during the Edo period.

Symbols of Japan: Thematic Motifs in Art and Design
Merrily Baird