Even though Koi are extremely hardy, it is very important to keep a watch on their environment to keep them at their healthiest. Regular observation of your Koi will allow problems to be detected early on but you should be familiar with your koi's normal shape, size and color as well as swimming habits. A change in these factors may signal that your Koi is stressed or ill and the problem should be identified in order to know what Koi medication or pond water conditioner to use for the treatment.
Speaking of pond water conditions remember that the koi's behavior could also change due to poor water conditions and you might need a pond water conditioner instead of a Koi medication. Attention should be paid to the quality of water and balance of the pond (i.e. pH levels, Ammonia, Nitrite etc.).
Unfortunately every pond owner with Koi at some time or another, will probably have to medicate their Koi pond with some type of Koi medication or pond water conditioner. Koi jumping and breaking the water surface, rubbing or scratching the pond sides and bottom could also indicate the need for a pond water conditioner or Koi medication.
In most cases it is easiest to test your pond water conditions and if they are in the normal range then you might have to medicate your Koi. A microscope is very helpful in diagnosing parasites and helping you decide what Koi medication to use.