05. Koi diseases – Diagnosing and Treating In Koi Pond

Unfortunately, it is a fact of life that disease will inevitably affect every koi or goldfish pond at some time – making it necessary for us to diagnose and treat the disease effectively. We have already seen that it is quite natural for koi to carry a low level of disease, being kept in balance by the fishes’ immune system.

However, should the worst happen and our koi become stressed, they will inevitably become susceptible and through the chain reaction discussed last month, will allow the resident opportunistic pathogens to flourish, and cause an outbreak of disease.

The golden rules for treating disease effectively are to treat your koi as quickly as possible with the most appropriate treatment having positively diagnosed and identified the disease. In addition, we should consider the disease not necessarily as the cause of the problem, but as the biological symptom of a stressed koi or an unbalanced pond.

By taking this approach, we are in effect treating the symptom (the flourishing disease organisms) and when doing so must also identify the factors that have stressed our koi in the first place. Therefore, treating disease is a two pronged attack: by using medication and identifying and rectifying the cause of disease outbreak.

Treating and Healing: A Team Effort.

If we treat disease without removing the stressors, our victory over disease will be short-lived. Furthermore, when we treat against disease organisms we are simply killing or reducing the number of disease organisms rather than treating the koi. We have to rely on the koi to heal itself, something it can only do under the influence of a stable and supportive environment and a nutritious diet.

Medications: Toxic but Effective

All medications are used for their toxic effects on the target disease organism, whether bacteria, fungi or parasites. Unfortunately, most medications are also toxic to our koi and may also retard filter bacteria and pond plants.

The approach to chemotherapy (chemical treatment) in the aquatic environment is quite unique in that the water is usually treated instead of the animal. The chemical dose is determined by the volume of our pond rather than the size or number of koi. In this way, when using a long-term bath treatment it is necessary to know the precise volume of our pond rather than the size or weight of our koi in the pond.

This has many obvious benefits in that our valuable and cherished koi can be treated without being handled and on a koi farm potentially thousands of fish can be treated in a single action.

Know your pond volume.

If we are uncertain of the volume of our pond then we risk either under or overdoing with medication. It is better not to treat at all rather than under-dose as under-dosing stresses our koi and does not eradicate the target pathogen, perhaps even enabling future generations of that disease to become resistant to treatment. The widespread use of some medications has meant that some bacteria are resistant to antibiotics and some parasites are resistant to insecticides.

Treatment on an individual basis.

Koi and other large pond fish suffering from certain bacterial conditions are sometimes better treated individually with antibiotic injections. In contrast, in this situation it is essential to know the weight of the fish when dosing, with the cost of the treatment being directly related to the weight of the koi injected.

We must remember that pond treatments are effective because they are toxic chemicals that can kill fish as well as their target organisms. The key factor is treating with a dose that is sufficiently concentrated to kill the disease but not the host. If we overdose, then we are likely to lose our koi also.

Four types of pathogen.

There are 4 categories of pathogen (disease-causing organisms): viruses, bacteria, fungi and parasites.

1. Viruses: Very briefly, viruses cause diseases in koi such as koi pox, more recently KHV and are also responsible for SVC which has recently led to a koi farm in the USA being closed and disinfected. Viral diseases cannot be treated because of their mode of action and this is why most of the notifiable diseases in the UK are viral. Once a koi has contracted the disease, it is untreatable and down to the fish’s own immune response to attack the virus. (TEXT BOX: A notifiable disease is a disease listed by DEFRA whereby any occurrence of such a disease must be reported to DEFRA who control the movement of these fish and thus control the spread of the untreatable viral disease. Notifiable diseases are responsible for closing down many fish farms as a result of the strict practices that infected sites must follow.)

Koi, just like humans, can be vaccinated against certain viral diseases where they are inoculated with a weakened form of the virus that stimulates the body to produce anti-bodies against that virus. In future infections by that virus, the body has an advantage in that it already possesses the antibodies to attack the virus. The fish is then immune to that virus. Fish vaccination is more common in fish farming and can be carried out using a dip or bath but their efficacy is rarely regarded as being long term.

Bacteria. Bacteria are responsible for causing external complaints such as fin rot, gill rot, ulcers and ‘mouth fungus’ and are usually treated by dosing the pond water with a chemical treatment (formalin, acriflavine etc). The most common pathogenic bacteria in koi are Aeromonas (ulcers) and Pseudomonas (fin rot). Ulcers will generally respond better to topical treatment supported by a course of antibiotic injections to treat the systemic causes of infection. Prior to antibiotics being used on a koi, the bacteria must first be identified as well as the antibiotic that is most effective against it. This can be done by cultivating bacteria taken from a swab and subjecting them to an antibiotic sensitivity test. Other more specific internal bacterial complaints which may cause haemorrhaging and possibly dropsy are better targeted using just an injection of antibiotics. Antibiotics are only available under prescription from a Vet and injections are only really suitable for larger fish. Vets can also prescribe antibiotic food.

Fungus. Fungus is responsible for the cotton wool-like growths found on wounds or abrasions. Fungus is often quite difficult to treat, especially in advanced cases, as the protruding fungus growth is the tip of the iceberg in that out of sight, the fungus is penetrating deep into the living fish tissue. Unlike the other categories of disease, fungus is not contagious and affected koi will not infect other healthy and intact koi. Saprolegnia is the most common fungus to affect koi and other pond fish. It is particularly common up in spring and autumn, either side of a koi’s at natural period of inactivity and may even be colonised by algae, giving the fungus a green hue. The whole pond can be treated with malachite green and if an individual koi is suffering from a stubborn case of fungus it can be given a salt bath on a daily basis until the fungus recedes.

Parasites. Parasites are probably the greatest threat to koi compared to other treatable pathogens as they can prove difficult to control, particularly some of the larger parasites.

Single-celled parasites (Protozoa) are easy to treat and control with standard pond treatments (Formalin and Malachite Green). But because of their relatively simple structure, they can reproduce rapidly, causing our koi real problems in a short period. These parasites include a Trichodina, Whitespot, Ichthyobodo (you may know this as Costia) and Chilodinella. These ectoparasites attach themselves and feed off the skin and mucus, causing koi a great deal of irritation.

Larger, multi-celled parasites (Metazoa) include the microscopic gill and skin flukes but also much larger parasites that can be seen by the naked eye (Fish Lice, Anchor Worm) up to those several centimetres long such as leeches and tapeworms. These more complex parasites can prove more difficult to control with formalin and malachite green due to the high concentrations that are required to be effective. Salt baths work well for the larger parasites with the most difficult being the microscopic Skin and Gill Flukes (Gyrodactylus, Dactylogyrus).

Nature’s Balancing Act

Parasites on koi in mud ponds live in a finely balanced relationship where their level of infection does not cause the death of the host. It is in the parasite’s interests to keep its host alive. In intensively stocked koi ponds, however, koi and parasite relationships can become unbalanced causing the death of the host.


For the majority of diseases, it is possible to make a reasonably effective diagnosis as to the causative pathogen by observing the symptoms shown by your koi. For example, cotton wool-like growths will indicate a fungal problem, where as fin rot or lifted scales and ulcers can indicate a bacterial infection. Where koi repeatedly flick and flash through irritation, first check that the water quality is ideal and then look into a potential parasite problem. Many of the recent reports by koi keepers who have been unfortunate to see their koi exposed to KHV first treated their koi for the common bacterial and parasite symptoms. This virus however appears to make koi susceptible to these more common pathogens as well as causing koi to show more extreme symptoms each of appear not to respond to the standard methods of treatment.

Identifying microscopic pathogens using a microscope.

Where it is not possible to identify a disease with the naked eye, you will need to take a mucus sample and view it under a microscope to confirm the presence of a microscopic pathogen. Not only will this allow you to make a definitive diagnosis, but subsequent mucus scrapes will allow you to plot your koi’s progress under medication.

Take a clean, dry blunt spatula (or microscope slide if you are careful)

Net a koi and expose its dorsal surface above the water

Gently pull the slide from head to tail for 4-6″.

Reposition the mucus to the centre of the microscope slide and add a drop of pond water.

Compress the mucus sample gently under a glass cover slip.

View under x 40 and then x 100 to view parasites.

View under x400 – x1000 with oil immersion for bacteria

Identify the type of parasite and also their density in the field of view.

By taking mucus scrapes from healthy koi, you can soon get an ‘eye’ for what are acceptable and unacceptable levels of parasites on a koi.

Do’s and Don’ts when diagnosing and treating disease.


Know the accurate volume of your pond

Add additional aeration to the pond or bath during treatment

Get an accurate diagnosis before treating

Identify the cause of the stress/disease outbreak and put it right.


Treat first, diagnose later

Be tempted to add more treatment in ‘major’ disease outbreaks

Treat without first checking and recording your water quality parameters

Leave koi in a bath unattended. They can soon show signs of distress and may even jump out.

Kill blanketweed and string algae.