At almost the majority of ponds that I have been called out to, to assist with serious health problems, I found skin fluke.
Skin Fluke is one of the major killers of Koi. It is often not the fluke as such, but a fluke infection very often goes over into a secondary problem being a bacterial infection and consequently death.
There are two types, skin fluke and gill flukes.
The above photo represents a slide with skin fluke, which is by far the most common. In all the years of having Koi, I never had problems with gill fluke, although skin fluke does occur from time to time.
Neither one of the two is visible without a Microscope.
The first sign of skin fluke is normally a lack of appetite, build up of mucus and sporadic flashing. If you see the signs, you should immediately do a skin scraping to identify and treat it, before it explodes in numbers and go over into a bacterial infection. Often, the fish scrape against the walls to rid themselves of the fluke, and by doing that, creates open wounds, which can result in infection and death.
If you find one fluke on a slide, it does not mean that you have a big problem. Ideally you should have no fluke at all, but it is fine for Koi to have the odd fluke. However, if you find more than one on a slide, you have to treat immediately. The secret to the success of treatment of Koi is to be able to treat before the problem becomes to serious.
I normally have a black fish(Haijiro or Hageshiro) in my ponds. On a black fish you can see any built up of mucus easily and identify a problem in your pond before it creates problems. They, together with a white fish that shows stress(red on the base of the dorsal fin) easily are my early warning signals.
Should you find fluke on the slide, you have to treat the whole pond, because you can expect fluke on most of the fish.
There are various treatment options available.
A common treatment is to use malachite green and formalin combination (if you do not have a high solution of salt in your pond and your water temperature is above 12 degree Celcius)
There are also various medication designed specifically for fluke like Flukeaway and Prazi but they are quite expensive.
I personally always use Potassium Permanganate at 2,5g per 1000 liters and continue with the treatment two days apart until I am able to maintain a pinkish colour for at least 6 hours. It normally requires three or four treatments depending on the amount of organic matters present in your pond. It is a cheap and very effective treatment and at the same time addresses any possible bacterial problems caused by the fluke. It will also sort out almost all other parasites if present at the same time. It does affect your beneficial bacteria, but if you bypass your filter during treatment, you regain your beneficial bacteria very soon. An added advantage is that it leaves you with a crystal clear pond. One drawback of a PP treatment is that you have to do a 20-30% water change after each treatment and you should never overdose. Therefore it is extremely important to know the exact volume of your pond.