All animals have a built-in immune system that acts as protection from viruses and bacteria that cause sickness. Generally, this is good enough in warding-off disease, but when stress levels are high, this natural defense system can fail. This goes the same for Koi fish and disease is usually a result of too much stress caused by poor quality water, not enough food or drastic changes in water temperature.  

The common carp is known to be one of the hardiest species of fish, and this trait trickled down to the modern Koi. However, in their quest to bring out the most beautiful colors and patterns in Koi, breeders didn’t bother to develop their immune systems and as a result, Koi can be sensitive to abrupt changes in temperature and water quality.  

Koi can be stressed by high ammonia content in the pond water, the wrong ph level, lack of oxygen and not enough food. When these fish undergo continuous stress, they can succumb to parasites, fungus and bacteria that might be lurking in the water. Although most diseases caused by these things can be treated using salt, kanamycin and other medicinal treatments, it’s best to avoid this and concentrate on prevention.  

Flukes, trichodina and similar parasites are one of the major threats to the health of Koi. Considering you keep your pond water pristine and keep close watch on the health of your collection, the only way parasites can afflict your fish is by introducing new Koi into the pond. No matter how reputable the source of your new Koi is, it’s impossible to be 100% sure that they’re parasite-free. For this reason, new fish should undergo a proper quarantine process. They should be kept in a quarantine tank prior to introduction into your pond. The ideal quarantine period is at least 21 days and this also gives your new Koi time to recuperate from the stresses of being packed in a box and transported for several hours.  

Choosing a healthy fish from your pond and placing it in the quarantine tank with your new Koi will help it recover faster because these fish are used to being with other members of their species. This will also give you the chance to watch out for any differences in the behavior of your new fish with the healthy one from your Koi. Usually, a sick Koi will sulk and tend to stay in one area of the pond or tank.  

Koi disease can be spotted easily by watching your fish closely as they swim about. If they exhibit abnormal behavior such as scraping their sides against rocks or the pond wall, staying near the surface or aerators gasping for air, or swimming in an awkward position, this is a sure sign they have succumbed to parasites or fungal and viral diseases. There are several treatments available for sick Koi, but again, nothing beats prevention. Having test kits in stock to check ph levels, ammonia concentration and oxygen amount can go a long way in keeping your fish stress-free, happy and healthy in their home.