Generally, koi fish suffers from gastrointestinal diseases and some internal parasitic diseases, which will make him emaciated until he dies. In the early treatment scheme, gentamicin or furazolidone can be added with yellow powder and five thousandths of salt, and the sick fish can get better after a few days. If it is internal hair, you should take the medicine according to the specific situation. You can describe it in detail or send photos to help you have a look.