Domestic koi fish can keep three, five, six and nine. This kind of fish is an ornamental fish cultivated by carp, which has strong adaptability to temperature and miscellaneous feeding habits, so it is still very simple to raise. Moreover, it is gentle in nature and likes to swim in groups, but it has high requirements for oxygen. If the dissolved oxygen is insufficient, the fish's breathing will be affected, and even they will die of lack of oxygen.