Conversely we should not medicate to the point that the fish have nothing on them as anti-biotics and medications are indiscriminate in their action. Koi need specific micro-organisms to survive. Should the immune system fail and the fish become infected, the standard procedure is to treat with antibiotics. The unwitting person, in an effort to help diseased fish, may, in actuality, depress the immune system response even more by administering antibiotics. Certain antibiotics are known to interfere with the immune response in koi and increase bacteria resistance to treatment.
Environmental changes are stressful to koi and lower their resistance to infection and disease.
Experience has shown that a wide variety of bacterial, parasitic and other diseases become a problem only if koi are being held under environmental conditions that are unfavourable to them. The hobbyist must always keep in mind that fish diseases are usually not caused by any single event but are a result of the interaction between several 'agents'. Here are some examples of diseases and the stress factors that contribute to the infection. Theories on koi disease must always take into account the fact that in any koi community a large percentage of normal healthy individuals continually harbour potentially pathogenic microbes without suffering any symptoms themselves. Put another way there are always 'carriers' in any pond which have latent infections.