The best koi have a good balance of colors and patterns according to their variety. For example, there are koi with light blue spots, ones with large red patches on their backs, or all-metallic gold.

"These nishikigoi varieties are what westerners now refer to as koi fish. Koi became popular in Japan in the 1820s and then popular worldwide in the early 1900s after being part of an exhibition ...

Among the most difficult to save are those that live in water. David and Jane Pierce have owned — and loved — these koi fish for 25 years.Credit...Stella Kalinina for The New York Times ...