Overview Many things can compromise natural immunity. Koi are very hardy fish and have exceptional natural immune systems, which can heal most diseases without antibiotics. The first avenue of attack for the bacteria will be the skin or the gills. Their natural immune system is compromised by a whole host of things, including , but not limited to: Receiving Shipment  When you receive your Koi by mail or directly from the wholesaler, they will be weak, usually not fed for 5 days, sometimes treated with medicines or anesthetized, cooled down with ice and swimming in Ammonia laden water. These factors increase resistance to medicines and lessen natural immunity. If you then, do not quarantine the Koi under ideal conditions for 3 - 4 weeks, the natural immune system has no chance to regenerate.   Temperature  While we keep Koi at less than optimum Temperatures in our ponds,  a new arrival should be kept @ 22-24 degrees Celsius, 71.6-75.2 degrees Fahrenheit for at least 3 weeks, to allow the natural immune system to regenerate.   Oxygen  Maximum water circulation should be employed during this period, for oxygen absorption and CO2 removal. Adding straight Oxygen to the quarantine tank is not recommended.   Other factors Disease, poor prolonged water quality, poor diet and overcrowding.   Resistance to Medicines    The state of Niedersachsen Fish disease control and Fish Health Bureau in Hannover tested 4436 food and tropical fish since 1985 for reduced natural immunity when medicines are used. Of the 4436, 250 Koi groups were tested, which amounted to 500 Koi tested.   In the course of the tests a resistance system to medicines was developed with 0 being the least resistance to 100 being the most.   Chloramphenicol, Trimethoprim/Sulphonamid, Chlortetracyclin, Oxytetracyclin , Furazolidon. Enrofloxacin (Baytril®), Gentamycin were used in the tests. Food fish scored 40.2 Pond fish scored 64.0 Koi scored 64.7   From Aquaculture Health International: Yechiam Shapira and colleagues recently published a paper in the journal Aquaculture describing a series of experiments looking at the resistance of various crossbreeds of domestic and wild strains of carp (Cyprinus carpio) to KHV challenge. In laboratory trials, a domestic/wild crossbreed resulted in substantial resistance to viral challenge (60.7 percent survival). At the other end of the spectrum, one particular strain of domestic crosses was particularly susceptible to KHV, resulting in a postchallenge survival rate of only eight percent. Such results highlight the potential of selective breeding to reduce the impact of KHV in both ornamental and koi carp. For further information, consult Shapira Y,Magen Y, Zak T, Kotler M, Hulata G and Levavi-Sivan B (2005). Differential resistance to koi herpes virus (KHV)/carp interstitial nephritis and gill necrosis virus (CNGV) among common carp (Cyprinus carpio L) strains and crossbreds. Aquaculture 245. pp 1-11