Freshwater koi fish, also known as nishikigoi, are a popular species of ornamental carp that are kept in ponds or large aquariums. They are known for their beautiful and distinct color patterns, which include a variety of shades of white, black, red, orange, and yellow. Koi fish are social and intelligent, and they can even be trained to eat from their owner's hand. They can grow quite large, with some specimens reaching over three feet in length, so a spacious environment is necessary. Koi require a balanced diet and clean water, and they are often seen as a symbol of good luck and fortune in many cultures.