A special kind of koi fish. Mustard koi fish is a special koi fish variety, also known as "mustard carp" or "mustard fish". Their bodies are mustard yellow or mustard yellow with black spots, hence the name. Mustard koi fish is usually large in size and beautiful in scale, which is very popular in the ornamental fish market. They are koi fish varieties, which are artificially propagated. Mustard koi fish is particularly popular in Japan and is regarded as a symbol of luck and auspiciousness.