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The goldfish is not only one of the Germans' favorite pets, following the cliché. In reality, too, it populates numerous aquariums and garden ponds. Due to the limited water volume, the risk of goldfish diseases should not be underestimated. It is important to act quickly, especially when a fungus or other pathogen has found its way into the defined biotope. We explain how.

goldfish diseases

Before looking at individual diseases, it is worth taking a quick look at the reasons why the goldfish in the pond or aquarium is at an increased risk of becoming ill. Because where the danger is particularly high, changing the circumstances may reduce the risk of illness from the outset without having to take real measures against specific illnesses later. These aspects, individually or in combination, increase the risk for the goldfish of becoming ill from fungal infestation or other pathogens:

  • too intensive occupancy of the pond
  • Overfeeding or unsuitable food for the goldfish
  • Incorrect planting of the pond
  • Incorrect water parameters, e.g. temperature, pH value, oxygen content etc.

All of these things either mean that the goldfish is particularly susceptible to the various causes, or that existing pathogens can multiply excessively and thus also contribute to a significantly increased risk of infection. Looking at these things helps to effectively curb disease transmission and keep fish resilient and healthy.

If the goldfish falls ill despite good care and guidance, the diseases can usually be brought under control with early detection and the right measures.

semolina disease

  • Cause: Fungal infestation, transmitted by ciliates that settle in the gills
  • Symptoms: The skin of the fish is covered with small, white spots
  • Effects: Emaciation of the fish, constant chafing due to itching caused by fungi, secondary infections from chafed areas

A help against the semolina disease can be fungicidal agents that fight the fungal attack on the skin of the fish. However, it is more important to eliminate the cause, i.e. the ciliates causing the disease. This is achieved by using malachite green, but only during the swarming phase, when the ciliates leave their breeding ground in the epidermis of the goldfish gills and move freely in the water. A slight increase in the water temperature can accelerate the development of the creatures and cause swarming at an early stage.

notice: Early application of fungicides is essential in this disease. When the fish are scoured, parts of the fungi fall off and find ideal reproduction conditions on the pond or aquarium floor. The result is a rapid spread of the infestation and sooner or later the entire population will become ill!

fin rot

  • Cause: Saprolegnia mold, usually as a secondary infection of previous injuries or illnesses
  • Symptoms: Infestation of the caudal fin, degeneration and dropping off from the rear end, in the end usually only fin stumps remain
  • Effects: disturbed locomotion possibilities of the fish, usually only little assertiveness against conspecifics and sooner or later death

In order to get fin rot under control, the triggering factors should first be eliminated. These are poor water quality and stress on the fish. It makes sense to move the sick animal so that targeted treatment with antifungal medication is possible without burdening the healthy animals with the medication. If, on the other hand, numerous specimens are ill, application directly in the pond may also make sense.

notice: From a purely medical point of view, fin rot is not contagious. However, the triggering factors can lead to large numbers of animals becoming infected quickly, giving the visual impression of transmission.


  • Cause: Fungal infection of the internal organs, especially the swim bladder
  • Symptoms: staggering swimming movements, difficulty controlling direction and height of locomotion
  • Effects: rapid emaciation due to destruction of the stomach and intestines, rapid death

A remedy when goldfish staggering occurs is only possible in the early stages. In the case of advanced disease, however, the affected animal can usually no longer be saved. A sick goldfish should be isolated as soon as possible and treated with an antifungal agent in the pool water. At the same time, the pond water should be cleaned to remove the main carrier, faeces, from the water.

tip: Often the causes of dizziness lie in cheap, contaminated fish food. If you pay attention to the quality and origin of the feed, you can significantly reduce the likelihood of animals getting sick.

gill rot

  • Cause: Fungal or bacterial infestation, mainly of the gills, caused by too high population density and too high water temperature
  • Symptoms: fungal or bacterial growth in the form of yellowish-brown spots in the area of the gills, frequent gasping of the fish
  • Effects: damage and degeneration of the gills, death of the animals by suffocation

Both possible pathogens of gill rot, i.e. bacteria and fungi, are usually naturally present in the pond water. However, when the water quality deteriorates, they can multiply rapidly and pass on to the host animals in large numbers. If the disease occurs, the pond water should be treated with a drug that is usually effective against both pathogens, while improving the water quality prevents it from spreading. Isolating the diseased fish helps reduce stress and improve recovery.

leech infestation

  • Cause: parasite fish leeches
  • Symptoms: Abnormal behavior of the goldfish, caused by deep puncture wounds by the sucking leech, sometimes spots or bumps as a result of infection of the puncture
  • Consequences: frequent secondary infections through stab wounds and general weakening of the goldfish

To help the goldfish, the first thing to do is to catch it and remove the leech from the fish skin. If the leech does not let go, a bath in salt or lime water can make it easier to detach. To permanently banish the parasite from the pond, all fish should be caught and the pond drained. The pond sludge can then be easily treated with salt or quicklime. Both the actual leeches and their clutches die off within a short time when they come into contact with both substances. In addition, possible breeding sites, i.e. silting up bank areas etc., can be avoided as a preventative measure.

notice: It is true that many pond inhabitants, including goldfish, like some vegetation as cover and as a source of food. However, this should not be confused with an overgrowth and silting up of the edge zones!

Dropsy (ascites)

  • Cause: Viral infection, favored by strong fluctuations in water temperature and oxygen content
  • Symptoms: initially a punctate bump, later an overall bloated fish belly and protruding eyes
  • Effects: with moderate infestation usually without lasting effects, with intensive infestation weakening of the animals up to death

As with humans, it is not possible to take action against the causative viruses. Instead, you should provide the sick fish with the best possible environment for recovery. A high water quality and a low stress level help to strengthen the organism. The probability of infection can also be significantly reduced by these aspects. A kind of "time out" for the sick fish in the form of temporary isolation is particularly effective.

notice: The causes of ascites are not actually an infection, but are caused by malnutrition and other debilitating factors. Tissue fluid is then discharged into the abdominal cavity, leading to the named local bulge and later to the overall bloated abdomen. However, the occurrence of these consequences is significantly increased and even actively promoted by a virus infection that occurs again and again.

Anchor Worm (Lernaea)

  • Cause: parasitic anchor crab (!)
  • Symptoms: rod-shaped appendages on the fish body, up to 20 millimeters long
  • Effects: Weakening of the fish, usually followed by infections and further parasite infestation

Contrary to most other goldfish diseases, isolating infected fish with anchor worm makes no sense. Since the parasite resides in the entire pond, antiparasitic agents should be used in the actual pond water and not in the separate quarantine tank. The agents prevent the further development of the crustaceans and lead to their death. In addition, adding antibiotics to the pond water can prevent possible secondary infections at the bite sites.

notice: Although the parasite is commonly known as the anchor worm, it is actually a crustacean.

Skin, Gill and Tapeworms (Trematoda, Cestoda)

  • Cause: worm-like parasites that infect either the skin, gills or the inside of the body
  • Symptoms: skin worms visible as appendages, other infestations usually only detectable through weakening of the fish and abnormal behavior
  • Effects: weakening of the animals, damage to the affected organs up to and including failure, subsequent infections from bite wounds

Worm infestation is also treated in goldfish with classic worming treatments. The worming agents are usually added to the pond water and prevent the animals from developing further. Due to the high effectiveness, the worms, which are already quite slow in their development, can be controlled very well. In addition, antibiotics in the water help to protect bite wounds from infection in the event of a very strong worm infestation.

Non-specific goldfish diseases

Again and again it can be observed in goldfish that the skin or the fins show spots. These are usually caused by a temporary fungal infestation and disappear quickly. If, on the other hand, the animals are under constant stress or are weakened due to other circumstances, serious problems can quickly arise from these actually uncritical fungi and the fish actually becomes ill. You should therefore monitor your pond fish regularly in order to be able to take countermeasures quickly and in a targeted manner.

Immediate measure isolation

No matter what disease your goldfish is diagnosed with, immediate isolation of the fish is recommended in almost all cases. Since the medium of water enables the pathogens to spread far more quickly among the goldfish population, the risk of an epidemic is otherwise relatively high, regardless of the disease. In addition, the isolation of the infested fish also serves its own well-being, since treatment in a stress-free environment is far easier and usually takes effect much faster than it would be in the pond.

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