How to Treat Flukes in Fish

Gyrodactylus flukes under a microscope

Jessie Sanders

Fish flukes are tiny, microscopic parasites that can irritate a fish's skin and/or gills. There are four main genera of flukes seen in cultured fish that can present similar clinical signs. Also known as flatworms, this parasite commonly occurs in freshwater fish, especially those without proper quarantine protocols. Flukes can occur in both freshwater and marine fish but will require different treatment protocols. Proper treatment will depend on the genus of parasite, its reproductive strategy, and the aquarium water temperature.

What are Flukes?

Flukes are a common name for monogenean trematodes, or flatworms. They live on your fish's skin or gills and feed on the cells and mucus for survival, using suckers and grasping hooks to attach to the tissues. Some flukes reproduce through live births while others are egg layers. Correct determination of the genus of fluke on your fish is helpful for proper treatment, since only certain life stages are affected by medications. Flukes are very common on most cultured fish in low, subclinical levels and spread when the fish are stressed from poor water quality, poor diet, or improper quarantine.

Symptoms of Flukes in Fish

  • Missing scales
  • Hazy look to the skin
  • Flashing behavior
  • Lethargy
  • Decreased appetite
  • Sudden death

Causes of Flukes

Flukes are commonly found on many pet and cultured fish in very low levels. At such low, subclinical levels, healthy fish are able to keep the parasites from replicating at high speed and therefore do not require treatment.

If a fish is stressed, such as from poor water quality, other disease, capture, handling, or transport, the fish's immune system becomes suppressed and is not able to keep most parasites at bay. The flukes are then able to reproduce more favorably. Rate of reproduction depends on the water temperature; flukes will replicate very quickly in warmer water. Infestation levels will depend on how many fish there are to feed on and if the flukes are egg layers or live bearers. The fluke's life-cycle is much quicker in the live bearing species and so they will increase on the fish more quickly.

Diagnosis of Flukes

Flukes are diagnosed by a biopsy sample of the skin mucus and/or gills examined under a microscope. This is a very common procedure in any physical exam of a fish. No one can see flukes with the naked eye. Macroscopic fish parasites, those that can be seen with the naked eye, on pet fish include fish lice and anchor worms.

Low levels of parasites do not necessarily warrant treatment unless your fish are actively showing clinical signs. Some fish can be more sensitive to various parasites than others of their own species. Consultation with an aquatic veterinarian is highly recommended in order to obtain a correct diagnosis and appropriate treatment plan.

Treatment

Treatment for flukes will vary based on the water type (freshwater vs. saltwater), number of fish affected and aquarium size. Duration of treatment will depend on which genus of fluke is present and the aquarium water temperature. Egg laying flukes require longer treatment periods to ensure removing all life stages, and cooler water also will require longer treatment times. Consult with an aquatic veterinarian in order to ensure your diagnosis is correct and treatment is appropriate.

Depending on your situation, your veterinarian may recommend a medicated bath, oral medications, injections, or a combination of treatments. Severe infestations will respond fastest to injectable medication. Oral medication will depend on if your fish are eating or not. Bath treatments can destroy your carefully cultivated biological filtration if not administered properly.

How to Prevent Flukes

The best method of dealing with fluke infestations is to avoid them altogether. In order to prevent fluke infestations, provide your fish with a low stress environment, regardless of their setup. This includes good water quality and maintenance practices, feeding a proper, nutritious diet, and adhering to strict quarantine protocols for all new additions, plants and invertebrates included.

For a proper quarantine, use a completely isolated aquarium with separate equipment and filtration. Do not allow water to splash between systems and wash your hands between maintenance rotations. Depending on your fish species and water temperature, you need to enforce a four-to-six-week quarantine period for all new fish additions. This will allow you to identify diseases early and not infect a larger population. It is significantly easier and cheaper to treat just a few fish in the isolation aquarium than in a larger aquarium or pond.

Are Flukes Contagious to Humans or Other Pets?

Thankfully, monogenean flukes are not contagious to humans or other pets. They are strictly aquatic pests and cannot survive outside of the water or penetrate tough outer skin. Wash your hands both before and after handling fish or their water to ensure proper hygiene for you and your fish.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Noga EJ. Fish Disease: Diagnosis and Treatment. John Wiley & Sons; 2011.

  2. Roberts HE. Fundamentals of Ornamental Fish Health. Wiley; 2009.