Regular observation of your fish will allow problems to be detected early.
Being familiar with your fish's health, normal shape, size and color as well as swimming habits will help you to keep very healthy fish. A change in any of these factors may signal a problem. The problem must be identified in order to know the steps to take for treatment.
Though Koi are extremely hardy, it is very important to keep a watch on their environment to keep them at their healthiest. Attention should be paid to the quality of water and balance of the pond (i.e. pH levels etc.). However, if problems do occur, the first step is to remove the individual fish or decide whether to treat the entire pond.
It is sometimes difficult to be aware of a problem with a fish until its too late. Be aware of your Koi's behavior patterns so changes can be detected early. A change in your Koi's behavior is usually the first sign that that your Koi are stressed or ill.