Koi immune systems are very weak below 50 °F. At this temperature, they also stop eating and don’t regain their appetites until the waters warm in the springtime. Koi are very hardy and, with proper care, they resist many of the parasites that affect more sensitive tropical fish species. Koi have been accidentally or deliberately released into the wild on every continent except Antarctica. However, within just a few generations, they revert back to their natural dark black color. Interestingly, in many areas, koi are considered an invasive species and pests because they increase the turbulence of the water that they live in. When water becomes more turbulent, it becomes too dirty for animals to drink and reduces the presence of aquatic plants. Koi disturbance of waterways has actually caused quite a bit of damage to water habitats and caused many countries to try to eradicate them, though mostly unsuccessfully.
