Koi can live a very long time - usually 30 to 40 years, and some have lived over 70. They are quiet, calm fish that recognise the people who feed them and will swim to the surface to say hello. Some pet koi let their owners stroke them on the head.
Koi can grow surprisingly big. A well-fed pond koi can reach the size of a small dog. They are slow growers - they keep growing all their lives, but only by a few centimetres a year. The older a koi, the more chunky and round it becomes.
Different koi patterns have different names in Japanese. A red koi with a white belly is called 'Aka Sanshoku'. An all-white koi with a single red circle on its head is called 'Tancho' - named after the red-crowned crane, because they share the same red dot on top.
Koi are a popular gift in Japan. On Children's Day in May, families fly long fabric streamers shaped like koi - called koinobori - from poles outside their houses. The streamers swim in the wind, one for each child in the family. The idea comes from an old story about a koi that climbed a waterfall to become a dragon.