Doroike

Mud pond. Pond dug into the mud used to grow Nishikigoi. The mud has abundant natural minerals due to the snow melt that refills the mud ponds every spring. Because of the large area of the pond Nishikigoi can exercise more. Hobbyists release Koi to a mud pond to promote health and increase growth from spring to autumn. Koi are rarely kept in mud ponds over the winter because they cannot be fed under the ice and the Koi will continue to grow much faster in concrete tanks in the breeder's greenhouses.

(Courtesy of the excellent Book Kokugyo Vol II)