Diseases in pond Koi fish

Diseases in pond fish
Diseases in pond fish

The best proof of a healthy pond is vital to fish. Unfortunately, the reverse is also true: as soon as something is wrong in the pond, pond koi fish increases rapidly.

Therefore, in this post, we want to deal with the causes and symptoms of diseases, explain some, and give advice on prevention and treatment.


Let’s start at the very beginning: With the reasons for disease in fish. The husbandry conditions are causally involved in most diseases in pond fish.

Examples of this are:

  • Poor nutrition.
  • Insufficient water values.
  • too-small pond.
  • A too high stocking density.

The resulting stress then leads to a weakening of the defense functions of the fish. It increases the susceptibility to parasites, bacteria, and other pathogens. Another common reason is that newcomers bring in diseases.

Therefore, it is advisable to keep newly acquired fish in a quarantine tank, observe them for abnormalities, and only then put them in the pond when they are free of symptoms.

Attentive pond owners can recognize many diseases early if they watch their fish frequently. You will find it easier to spot abnormalities.


It is often not that difficult to identify diseases early – you have to know what to look for. First of all, there is a behavior change: for example, a lack of escape reflex, lack of appetite,

standing around for a long time on the surface, or lying on the ground. Swimming disorders can be identified easily, such as staggering and standing upside down.

Rubbing against the ground or the edge of the pond and swimming forward are more likely to be dismissed – but these behaviors are often signs of illness.

Likewise, fish are occasionally plagued by itching and jump out of the water.

Changes in breathing are often more difficult to assess: rapid gill movements are difficult to detect in a regular pond, while emergency breathing on the water surface is more manageable.

In addition, diseases can lead to physical changes, which can occur in many different ways.

These can be color changes, deposits on the skin’s surface, emaciation, or changes in body shape. Our list here does not claim to be complete.

Because, of course – depending on the disease – other symptoms can also become noticeable.

Important: Many pathogens can multiply and spread quickly in water. So if you notice the first symptoms of the disease, react immediately!


Depending on the disease, you can also treat your fish independently, for example, with salt baths or over-the-counter remedies from pet shops.

A significant partial water change often helps. A diagnosis that is as accurate as possible is essential when treating diseases! Because even if there are drugs that are effective against several different conditions, there is no such thing as one broad-spectrum drug “against everything.”

And unnecessary drug treatments only put additional strain on the organism of your fish and may lead to unwanted resistance.

We, therefore, advise you to consult a veterinarian who specializes in fish in the event of illness. He can help your fish with targeted treatment and provide you with expert advice.

Common diseases in pond koi fish

Here are some key examples of fish diseases and their treatments. If you suspect an illness, we recommend that you seek the advice of a veterinarian who specializes in fish before treatment.

In this way, a precise diagnosis can be made and correct treatment initiated—unnecessary and incorrect therapies to be avoided at all costs for the benefit of your fish.

White spot disease ( Ichthyophthirius multifiliis )

This unicellular parasite causes typical white spots on the mucous membrane of its hosts. Occasionally the eyes of the fish are also affected.

Gill damage caused by white spot disease leads to shortness of breath.

The complicated Latin name of the unicellular organism is often used in abbreviated form (“Ichthyo”). Ichthyo is multiplying at an explosive rate.

The pocky white growths fall off the fish from time to time and the bottom.

After about 24 hours (depending on the water temperature), up to 1000 free-swimming swarmers emerge there, which infect the fish again. The following applies to treatment: the sooner, the better.

Therapy with malachite green, for example, is possible but must be carried out for at least (!) 5 days. Sometimes a more extended treatment period is necessary.

Costia ( Ichthyobodo necator )

These cloudy skin are classic parasites of weakness. In adult fish with a healthy and functional immune system, the unicellular organisms hardly have a chance of causing damage.

However, if the pond dwellers are still very young or already weakened by other diseases, these flagellates have an easy job. The drop in water temperature to below 15 ° C also promotes infestation.

The parasites then irritate the mucous membrane. Therefore, white-bluish shimmering opacities form.

By damaging the mucous membrane, they pave the way for additional infections, such as fungi. Therefore, massive infestation often leads to death. Sometimes salt baths are enough for treatment.

They support the metabolism of the fish and thus also stabilize the immune system. In any case, research is to be carried into the cause carried out before drug treatment.

Because when it comes to Costa Rica, it is essential to treat it and find and eliminate the reason for the immune deficiency.

Otherwise, you will never be able to finish off the parasites and only provoke resistance. If you have any questions, it is better to contact your trusted fish veterinarian.

Flukes ( Gyrodactylus spp., Dactylogyrus spp. )

These tiny little worms can be very annoying to your pond dwellers. As a rule, they cannot be seen with the naked eye. It can only be reliably detected with a microscope.

A distinction is made between skin eye worms (Gyrodactylus spp.) And gill eye worms (Dactylogyrus spp.).

The skin suction worm is mainly found on the outer skin.

It damages the mucous membrane layer and is a pioneer for other problems: Inflammation of the scaly pockets, algae, and fungus are possible consequences.

Affected animals occasionally scrub or jump, their skin may appear cloudy. Gyrodactylus gives birth to live young animals and, unlike Dactylogyrus, does not lay eggs.

The egg-laying gill suctionworm is mainly – but not exclusively – to be found on the gills. Affected fish have breathing problems because the gills are irritated and swollen by the infestation.

There are adequate preparations against flukes. If the infestation is low, simple salt baths often help.

Once the diagnosis has been made with certainty, you can use products from pet shops (be sure to read the package insert!) Or medicines prescribed by the vet.

Unnecessary antiparasitic treatments to be avoided at all costs. Otherwise, resistance will get promoted!

Carp louse ( Argulus sp. )

Contrary to what the name suggests, carp lice are crustaceans. Up to around 13 mm in size, these parasites can also be easily seen with the naked eye.

Waterbirds often introduce them. They sit tightly sucked on the skin and the flake pockets. Possible consequences of an infestation are reddening of the skin, caused by bleeding or inflammation.

There is usually massive itching. Affected fish, therefore, scrub themselves, for example, or shoot through the pond entirely suddenly.

Individual carp lice can be collected. If the infestation is severe, it must be treated with medication. There are over-the-counter remedies available in stores or medicines from your fish-savvy veterinarian.


Fish mold ( Saprolegnia parasitica )

This pathogen is virtually ubiquitous. As a rule, it cannot harm healthy fish with a functioning immune system. It gets dicey when the immune system is weakened,

for example, due to low temperatures in winter. Lesions in the fish’s mucous membrane (e.g., due to itching and the resulting chafing) also allow a Saprolegnia infection.

Especially long-standing and already infected wounds are also often overgrown by this fungus.

It typically manifests itself in cotton-like coverings. They are initially whitish but can also take on a greenish-gray color. Incidentally, the clutches of fish are, particularly at risk.

Here, fish mold regularly leads to significant losses.

You can remedy a fish mold infestation with commercially available remedies containing malachite green. More full short-term salt baths often bring relief.

Bacterial diseases

The appearance and course of bacterial diseases in the pond are very varied. There are almost no bacteria, the presence of which inevitably leads to infection in the fish.

Much more often, bacterial diseases are caused by bacteria found in the pond anyway and usually do not cause any damage.

If the stable system “derails,” these bacteria lead to diseases when they multiply on a massive scale. The conditions that result from this are usually named according to their symptoms.

“Hole sickness”

The “hole sickness,” also erythrodermatitis, is usually caused by bacteria. But other pathogens and – as so often – unsuitable environmental conditions also play a role.

Affected animals show large, ulcer-like holes in the skin. These usually sit on the trunk or a bit further back towards the caudal fin.

Sometimes you can look down at the muscles of the diseased fish. Depending on the trigger and environmental conditions, the disease progresses rapidly.

Sudden deaths and heavy losses are possible. It is advisable to consult a veterinarian who specializes in fish as soon as possible.

With the help of a smear, he can determine the pathogen, carry out a resistance test and initiate suitable therapy.

“Fin rot”

Frayed fins, milky-cloudy or red discoloration on the fin edges: This is what “fin rot” looks like. The generalized occurrence of this disease suggests suboptimal housing conditions.

Occasionally, individual fish are only locally affected; an injury is often the cause. Veterinary examination and treatment are also absolutely advisable for this disease.

Because other pathogens can also play a role, a detailed diagnosis should be made before deciding on a particular treatment method.

Because without eliminating the actual cause and improving the housing conditions, fighting the disease is not possible.

Viral diseases

Koi herpesvirus (KHV)

This disease has been described for about 20 years: the infection with the Koi herpes virus. It is a notifiable animal disease.

The most noticeable symptom of this disease is massive damage to the gills. However, the virus also affects other organs, such as the skin, intestines, and kidneys. The fish get infected inconspicuously.

As a rule, only stress in connection with temperatures between 16-28 ° C causes the disease to break out. 

Skin damage can occur. The animals are generally more susceptible to other diseases. Most noticeable is the sometimes massive shortness of breath caused by the gill damage.

For air, The fish stand on the water’s surface or the filter vent. Mass deaths can occur. A causal treatment is not possible.

For sick fish, all that remains is to optimize the keeping conditions and keeping them in isolation.

For the PCR-based detection of the Koi herpes virus, the veterinarian takes a small tissue sample from the gills.

Other fish (except for the carp) in the pond do not get sick, but they can transmit the virus.

Carp pox (CHV-1)

If the temperatures in the pond drop, you can occasionally see them: carp pox or koi pox. They appear as whitish, translucent, waxy deposits on the skin or the fins.

The small growths in infected animals always grow when the immune system weakens, especially when the environmental conditions are poor and the water is too cold (<12 ° C).

A “cure” in the strict sense of the word is not possible because the affected fish are constantly carrying the virus.

But smallpox may go away. 

As a rule, koi pox is not a cause for concern, and they are just blemishes. Only in extreme and sporadic exceptional cases do they cause serious harm.

Other diseases

Swim bladder infection

Inflammation of the swim bladder is primarily characterized by the fact that the affected fish can no longer provide buoyancy in a controlled manner.

In the worst case, the animal is constantly lying on the bottom of the pond. It can only move up and down with difficulty with fin power.

This expenditure of force consumes it, and it also soon develops callouses. Usually, only individual animals are affected.

Parasites, bacteria, or metabolic disorders are usually involved. The appropriate treatment method results from the cause and must be determined by a qualified veterinarian.

Regardless of the cause of the swim bladder infection, increasing the water temperature to around 25-27 ° C and adding iodine-free table salt to support kidney function usually help.

Unfortunately, once sick animals tend to have swim bladder problems again.

Energy Deficiency Syndrome (EMS)

The energy deficiency syndrome is a classic spring illness. It occurs when there is a deficit between the required and available energy.

Possible reasons for this lack of energy can be an inadequate diet in the summer months or too early feeding in the winter.

A low oxygen concentration in the water also favors the development of EMS. Emaciated, emaciated animals are logically particularly at risk.

Paradoxically, the energy deficiency syndrome also occurs comparatively frequently in obese animals – because they cannot use their fat reserves at low temperatures.

The fish affected by EMS show an uncoordinated swimming behavior, enormously slowed reactions, and shallow breathing.

Your kidney function is severely restricted, which causes water to accumulate in the body cavity. Affected animals sometimes appear thick and swollen.

The scales can protrude like a pine cone. The eyes protrude. Animals affected by EMS are helped by slowly heating the water by no more than 2 ° C per day and adding moderate salt.

A highly digestible feed can be started when the fish shows almost normal behavior again. EMS is an emergency! Deaths are not uncommon.

Prevent fish diseases

Prevention is better than cure! Because, as already mentioned, the disease can quickly spread in the pond and sometimes even be fatal. Offer your optimal fish housing conditions to prevent this.

Make sure that the water quality and filtration are good. The size of the pond should match the number of fish kept and the demands of the species you keep.

Eat a balanced diet. Store the food in a cool, dry, and air-protected place. So it is protected against premature spoilage. If necessary, supplementing the ration with vitamins can be helpful.

Also, make sure to find a knowledgeable fish veterinarian. If the worst comes to the worst, you should already have his contact details and not have to look for a suitable veterinarian.

We advise against the preventive treatment of your fish with pharmaceuticals. Unnecessary treatments stress the sensitive fish organism and may even lead to resistance in the pathogen.

It is essential to avoid this! Preventive examinations, on the other hand, really make sense.

Many fish veterinarians offer spring and fall check-ups. In the critical transition phases, you have a better overview of the health status and thus a higher level of security.

On the other hand, you can and should regularly check the water values ​​yourself. If there are changes to the negative, you can take countermeasures early on.

Active aeration of the pond using air pumps or the filter vent brings oxygen into the water. It helps the fish to cope better with sudden stressful situations.

Refrain from significant changes at low or low temperatures – as long as they are not necessary. 

Always keep an eye on your fish. In this way, you learn a lot about their natural behavior and recognize symptoms of illness all the more quickly.


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