Diseases in Pond Fish

The best proof of a healthy pond are vital fish. Unfortunately, the reverse is also true: as soon as something is wrong in the pond, the risk of disease in pond fish increases rapidly. In this post, we, therefore, 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, poor water parameters, a pond that is too small, and a stocking density that is too high. The resulting stress then leads to a weakening of the defense functions of the fish. This increases the susceptibility to parasites, bacteria, and other pathogens. Another common reason is that diseases are brought in by newcomers. It is therefore advisable to first 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. Vigilant pond owners can spot 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 just have to know what to look for. First of all, there is a change in behavior: 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 such as staggering and standing upside down can also be easily identified. Rubbing against the substrate or the edge of the pond and swimming forward are more likely to be dismissed – but these behaviors are also often signs of illness. Likewise, fish plagued by itching occasionally jump out of the water. Changes in breathing are often more difficult to assess: very rapid gill movement is difficult to detect in a normal pond, but emergency breathing on the water surface is easier. In addition, diseases can lead to physical changes, which can occur in many different ways. These can be changes in color, deposits on the surface of the skin, emaciation, or changes in body shape. Our list here does not claim to be exhaustive. 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 illness, 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 large partial water change often helps. A diagnosis that is as precise as possible is important when treating diseases! Because even if there are drugs that are effective against several different diseases: 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 resistances. 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 fish diseases in the pond

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 treatments should 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 single cell is often used in abbreviated form (“Ichthyo”). Ichthyo is multiplying at an explosive rate. The pocky white outgrowths fall off the fish from time to time and to the bottom. After about 24 hours (depending on the water temperature), up to 1000 free-swimming swarmers emerge there, which infest the fish again. The following applies to the 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 longer 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 have little chance of causing damage. However, if the pond inhabitants are still very young or already weakened by other diseases, these flagellates have an easy time of it. 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, for example with 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 into the causes should be carried out before drug treatment. Because when it comes to Costa Rica it is not only important to treat it, but also to find and eliminate the reason for the immune deficiency. Otherwise, you will never be able to finish off the parasite and only provoke resistance. If anything is unclear, 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. They 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 and is a pioneer for other problems: Inflammation of the scaly pockets, algae, and fungal infestation are possible consequences. Affected animals occasionally scrub or jump, and their skin may appear cloudy. Gyrodactylus gives birth to live young animals and, unlike Dactylogyrus, does not lay eggs.
The egg-laying gill lipworm 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 effective preparations against flukes. If the infestation is low, simple salt baths can often help. Once the diagnosis has been made with certainty, you can use products from specialist pet shops (be sure to read the package insert!) Or use medication prescribed by the veterinarian. Unnecessary treatments with antiparasitic drugs must be avoided at all costs. Otherwise, resistance will be promoted!

Carp louse (Argulus sp.)

Contrary to what the name suggests, carp lice are crustaceans. These parasites, up to around 13 mm in size, can also be easily seen with the naked eye. They are often introduced by water birds. They sit tightly sucked on the skin and on the pelvic 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 suddenly through the pond. 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 almost ubiquitous. As a rule, it cannot harm healthy fish with a functioning immune system. Things get dicey when the immune system is weakened, for example, due to low temperatures in winter. Lesions in the mucous membrane of the fish (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. This typically manifests itself in cotton-like coverings. They are initially whitish, but can also take on a greenish-gray color. Incidentally, the clutches of the fish are, particularly at risk. Here, fish mold regularly leads to large losses.
You can remedy a fish mold infestation with commercially available remedies containing malachite green. Highly concentrated short-term salt baths often bring relief.

Bacterial diseases

The appearance and course of bacterial disease processes in the pond are very varied. There are almost no bacteria, the presence of which inevitably leads to disease in the fish. Much more often bacterial diseases are caused by bacteria that are 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 diseases that result from this are usually named according to their symptoms.

“Hole sickness”

The “hole-in-the-hole” disease, also known as 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 little further back towards the caudal fin. Sometimes you can look down to the muscles of the diseased fish. Depending on the trigger and environmental conditions, the disease progresses at breakneck speed. 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 is also absolutely advisable for this disease. Because other pathogens can also play a role. Therefore, before deciding on a particular treatment method, a detailed diagnosis should be carried out. Because without eliminating the actual cause and without improving the housing conditions, fighting the disease is not possible.

Viral diseases

Koi herpes virus (KHV)

This disease has been described for about 20 years: the infection with the Koi herpes virus. This 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 can be infected inconspicuously. As a rule, only stress in connection with temperatures between 16-28 ° C causes the disease to break out. There is apathy and a lack of appetite. Skin damage can occur. The animals are generally more susceptible to other diseases as well. Most noticeable is the sometimes massive shortness of breath caused by the gill damage. The fish stand on the surface of the water or the filter vent and literally gasp for air. Mass deaths can occur. A causal treatment is not possible. All that remains for sick fish is to optimize the keeping conditions and keep 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 farm carp) in the pond do not get sick but 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 constantly carry the virus. But it is possible that smallpox will go away. This can be achieved by optimizing the housing conditions.
As a rule, koi pox are not a cause for concern, they are just blemishes. Only in extreme and extremely rare exceptional cases do they cause serious harm.

Other diseases

Swim bladder infection

Inflammation of the swim bladder is mainly 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 callous calluses. 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 energy needed and the energy available. 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 often in obese animals – because they cannot use their fat reserves for themselves at low temperatures.
The fish affected by EMS show uncoordinated swimming behavior greatly slowed reactions and shallow breathing. Your kidney function is severely restricted, causing 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 can be helped by slowly heating the water by no more than 2 ° C per day and adding moderate salt to the water. The 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 fish optimal housing conditions to prevent this. Make sure that the water quality is good and that it is adequately filtered. 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, it can make sense to supplement the ration with vitamins.
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 first.
We advise against the preventive treatment of your fish with medicinal products. Unnecessary treatments put a strain on 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 at an early stage. Active aeration of the pond using air pumps or the filter vent brings oxygen into the water. This helps the fish to survive suddenly occurring stressful situations better.
Avoid major changes at low or low temperatures – as long as they are not absolutely necessary. This also means that no newcomers should be used.

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 easily.

Mary Allen

Written by Mary Allen

Hello, I'm Mary! I've cared for many pet species including dogs, cats, guinea pigs, fish, and bearded dragons. I also have ten pets of my own currently. I've written many topics in this space including how-tos, informational articles, care guides, breed guides, and more.

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