Maruten

Round Hi pattern on the head of a Koi. When a Koi has only one Hi plate, and it is a round Hi plate on the head, the Koi is called a Tancho. When a Koi has multiple Hi plates including one round Hi plate on the head, it is referred to as Maruten. The round Hi on the Koi's head resembles a Japanese crane called a Tancho Zuru (crane).

The pattern also resembles the Japanese flag, and Tancho Kohaku are very desirable for this reason. Large round Hi patterns that stay on the head are most desirable. Small, round patterns on the head are called Ko (small) Maruten.

(Courtesy of the excellent Book Kokugyo Vol II)