Goldfish Parasites

Updated November 21, 2016

Parasites are small, harmful organisms that cause serious and often fatal infections in goldfish. In addition to the damaging effects these parasites have on the fish's gills, skin and internal organs, some parasites also carry harmful bacteria that attack the fish and cause additional complications. The treatment options vary depending upon the type of parasite.

External Parasites

External parasites occur on the outside of the fish's body. Anchor worms, for example, are between 0.20 and 0.30 inches long, with greenish, white or brown bodies. They appear as small threads hanging from the fish's sides. Infested fish may rub or scratch against the sides of the tank and often have red spots. Fish lice are disk-shaped crustacean parasites that attack the goldfish's skin, causing itching and ulceration. Both anchor worms and fish lice are large enough to see without a microscope. Flukes, which are wormlike parasites, carry several types of bacteria that cause severe ulceration.

Protozoan Parasites

Protozoan parasites are single-celled organisms that attack the goldfish's tissues and cells, causing diseases. Chilodonella, a heart- or leaf-shaped parasite, attacks the skin and gills. Chilodonella infections are characterised by cloudy or mucous-coated skin, folded or clamped fins, lack of energy and difficulty breathing. These infections usually occur because of poor water quality. Young fish are more susceptible to Chilodonella and are more likely to die from the infection than older fish. Costia, another protozoan parasite, causes milky or cloudy skin.

Other Protozoan Parasites

Ich or Ick (Ichthyophthirius multifiliis) is characterised by white spots on the goldfish's gills and body. Infected fish may rub up against objects in the tank. As the disease progresses, the fish may lose its appetite, have difficulty breathing and eventually die. Trichodina live in the water and travel from fish to fish. They feed on bacteria on the fish's body, causing lesions and weight loss. Infested fish may come up to the surface and gasp for air.


Reduce parasitic outbreaks by maintaining a high level of water quality. Protect goldfish communities from infestation by quarantining new fish and plants for several weeks before introducing them to the tank. Separate diseased fish and treat infections in a quarantine tank. Some parasites can be killed by raising the water salinity level. The Pond Doc website recommends adding 1.36kg. of salt to every 100 gallons of water to kill Chilodonella and Ich. Medications such as malachite green, potassium permanganate and formalin will kill other types of parasites depending upon the variety.

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