White-spot, Argulus & Chilodonella in koi pond the problem with guessing

  Case Histories

Case details

2nd April 2000:  The client phoned to say that over the last few weeks he had lost 9 koi. The fish that were left looked unhappy and one had a “lump” on its side.

(I think you will agree it wasn’t a lot to go on! Yet plenty of people would have been prepared to make a diagnosis and suggest treatment plans on this scrap of information).

The pond was a 20,000 litres (4,500 gal. UK) rectangular koi pond with a Cloverleaf filtration system.. The pond had extensive areas of frothy foam over the surface. The fish were nervous, several had reddening on the body and fins, some had fin rot and one had a severe swelling on its side with clearly visible raised scales.

The owner told me that six weeks ago somebody had been to see the fish, and after looking into the pond had announced that the fish had “mouth fungus” – sold the owner some treatment and left. Since that time fish had been slowly dying, with a total loss of 9 koi over the six-week period.

Water testing

Water tests showed a raised ammonia level (0.1 mg/litre) and a high nitrite level (1.0 mg/litre). This was traced back to the fact that the pond hadn’t been topped up for a while and the water level in the filter was only just above the chamber weirs – consequently  there was hardly any water passing through the filter. In essence there was hardly any filtration taking place – luckily it wasn’t summer-time when the build-up of nitrogenous wastes would have been even higher.

The filter had not been cleaned out for some time and was clogged with rotting organic matter – because the owner had been advised never to clean it. 

Fish examination

A skin scrape of two fish showed very high levels of white spot; indeed the slide was alive with them. There was also Chilodonella present and I found a couple of fish lice on the fins. The  koi with the “lump” was a mess when we took it out the water – the owner was shocked.

[It is important to realise that viewing the fish in water does tend to mask the degree of damage. This is why fish need to be examined out of the water]

There was an extensive area of swelling from the anus reaching forward to just before the pectoral fin. The scales were raised, with a large central area that was bloody and infected. There was also extensive fin erosion and indications that the bacterial infection had become systemic. I suggested to the owner that the fish should be euthanased, but he asked for it to be treated with antibiotics. I did treat it but its condition deteriorated and it had to eventually be put to sleep with an overdose of anaesthetic.

Treatment plan

All treatments were complicated because the  water temperature was only  80C, making treatment of any secondary bacterial infections difficult. Low water temperatures will also slow down the life cycle of both white spot and Argulus. For example,  the life cycle of white spot can be  four weeks or longer at these low temperatures. The problem being that the white spot parasites can’t be killed until they reach the free-swimming stage of the cycle.

I judged the white spot to be the main problem and the pond is now being treated with malachite green and formalin every 5-6 days. The frequency will be increased as temperatures warm up.

Once this problem has been resolved (hopefully within the next 5-10 days  now the water is warming up) an assessment will be made regarding the necessity of other treatments. One koi seemed to be affected more than the others. He had pink rather than white skin as well as frayed fins – a typical sign of severe stress. A close examination showed that he was covered in characteristic white spots. He was taken away and put in a heated treatment tank to speed things up. After just a couple of days in M&F he is looking much brighter. Hopefully there will not be any secondary problems.

Up to date

06/04/00: The pond has now had two treatments (M&F) – there have been no further losses. With a change of weather and warmer temperatures hopefully the next week or so will see an improvement.

Update 12/04/00: No further losses. Fish examined and skin scrape taken. No sign of parasites. Pond re-treated with M&F. Client advised to obtain some masoten to treat lice as water warms up. There doesn’t appear to be any secondary bacterial problems at present.  Because of the cold weather and a slower life cycle it is not possible to say for certain that the white spot has been eliminated, so a follow-up examination will be done in 2-3 weeks

Update 28/04/00: A follow-up examination showed that the white spot has been eliminated. The client has obtained some masoten and 2 treatments have been given. There is no sign of fish lice and there have been no further fish losses. A small amount of Trichodina was found and this will be treated in the next week or two. The fish that was taken away has been returned looking the picture of health.


Clearly, the cause of the original losses was a misdiagnosis or more accurately a wrong guess. What started with a simple parasite problem has now been complicated by secondary problems.

These problems have been complicated by failure to maintain the system and make sure the filter was working properly. The spread of the disease has also been helped by typical British spring-time weather with fluctuating temperatures.

The low water temperatures have meant modifying the treatment regime to take account of the extended life cycle of the white spot parasite – a calculation that could not have been made without a skin scrape to actually identify if and what sort of parasites were present! 

Once a proper examination was carried out the problems were easy to deal with. However, if a guess had been made and a couple of random treatments used there is every possibility that there would have been further losses and complications in the weeks to come.