This material is from the 4th edition of The Zebrafish Book. The 5th edition is available in print and within the ZFIN Protocol Wiki.


Fish Diseases

(Source: C. Walker)

For any large colony of fish, precautions should be taken to prevent the spread of epidemic disease. The easiest strategy for combatting disease is prevention by minimizing contact between fish and water in different tanks. Avoid mixing fish from different tanks as much as possible. Sterilize all equipment that comes in contact with the fish or tanks. For example, use fish nets, siphons, and cleaning sponges on only one tank at a time and autoclave them before using them in a different tank. Sterilize the water (i.e. with a flow-through ultra-violet sterilizing lamp) before adding it to the tanks. Remove sick fish from tanks as quickly as possible. Quarantine fish from pet stores before adding them to the colony (see Quarantine Room Procedures, page 1). Wash hands and arms thoroughly if they come into contact with tank water. The two most common diseases that affect zebrafish are velvet disease and fish tuberculosis (mycobacteriosis).

Velvet disease

Zebrafish are highly susceptible to velvet disease, Oodinium pillularis, a parasitic dinoflagellate algae. This oval-shaped parasite attaches to the fish near the fins, especially the dorsal fin, and around the gills. You can see it under a dissecting microscope. When the parasite is mature, it drops off the fish and multiplies 60 times on the bottom of the tank. These new parasites then reinfect the fish. Fish with velvet disease have the characteristic behavior of rubbing their sides and flipping around in the corners of the tank. As the disease progresses, fish become lethargic, the fins (particularly the dorsal fin) are held close to the body, and the fish stop producing eggs. Most fish from pet stores or dealers carry velvet disease.

Symptoms of velvet disease:

Although velvet disease is extremely contagious, it can be cured with minimal damage to the fish using a 3-day treatment with Atabrine (Quinacrine hydrochloride).

Treatment for velvet disease:

  • Day 1
    • Turn off incoming water.
    • Slowly drip 2 liters of sea salts into an infected 10-gallon tank.
    • Add 3.3 ml of the Atabrine stock solution.

  • Day 2
    • Add 3.3 ml Atabrine stock.

  • Day 3
  • Add another 3.3 ml Atabrine stock for a total of 9.9 ml.
  • At the end of the 3-day period, clean the bottom of the tank thoroughly and slowly dilute out the salt and the Atabrine with fresh water.
  • Continue cleaning the bottom of the tank daily for several days.
  • Solutions:
    • Atabrine Stock:10 mg/ml dH2O. Store in light tight bottle.
    • Salt Stock: 20 tablespoons (280 g) Instant Ocean Sea Salts (Aquarium Systems, Inc.) dissolved in 2 liters distilled water


    Fish tuberculosis or mycobacteriosis is also a common disease. The agent, Mycobacterium marinum, is a rod-shaped, gram positive, non-motile, non-spore-forming bacterium1. Fish are infected by ingesting the bacteria in unpasteurized food or through abrasions in the skin. Snails often harbor the bacteria.
      Symptoms of Mycobacteriosis:
      • listlessness; the fish may isolate itself from other fish and refuse to eat
      • emaciation; hence, the name "skinny"
      • skin ulcers
      • spinal curvature
      • pigment change
      • grey-white nodules in internal organs
    Definitive diagnosis should be made by a pathology lab. There is no known effective antibiotic to treat mycobacteriosis. Some level of disease control can be obtained by eliminating sick fish, by sterilizing tanks routinely and all equipment that comes in contact with the fish or the tank water, and by reducing stress caused by moving fish between tanks or by changes in temperature, water flow, or feeding regimen.

    A word of caution regarding fish tuberculosis: Some cases of transfer of fish tuberculosis to humans has been documented. Precautions should be taken by wearing disposable gloves and washing hands with betadine (providone iodine) when handling sick fish. Persons with immuno-deficiency problems should not handle fish with TB.

    Other diseases that occur less frequently include raised scales, tumorous eyes, and hemorrhages. In these instances, remove the diseased fish as soon as they are seen and clean the tank.

    Reference: Post, G. (1987) Textbook of Fish Health, T.F.H. Publications, Neptune City, NJ, pp. 228.

    Intestinal Capillariasis

    (Source: M. Pack, J. Belak, C. Boggs, M. Fishman and W. Driever)

    Capillarids are thin and transparent worms that can reach one centimeter length. The eggs have a characteristic oval shaped appearance with a plug-like structure at either end and are visible in the adult worm, and the gut or feces of infected fish. Photographs and a good description of Capillarids at different developmental stages can be found in the Handbook of Fish Diseases, Dieter Untergasser, Editor (TFH publications, 1989, p. 104-105). Worms in the intestinal bulb of adult zebrafish are motile and can be easily seen when the dissected gut, in egg water (0.03% Instant Ocean), is viewed with transmitted light using a high power (50X) dissecting microscope. Outside the fish gut Capillarids are no longer motile.

    A combination of two drugs, Trichlorfon and Mebendazole ("Fluke-tabs"; Aquarium Products, 180-L Penrod Court, Glen Burnie, M.D. 21061) has been reported to be extremely efficacious for removing monogenetic trematodes from fresh water fish. Trichlorfon is an insecticide with anti-cholinergic activity that is considered toxic to humans. Mebendazole, a common anti-helminthic used to treat human intestinal infections, inhibits glucose uptake and is cidal for adult helminths and embryos.

    Use the dosage recommended by the supplier; one tablet per 38 liters once trials to assess toxicity are completed. Add the drugs as a slurry (100 tablets per one liter water-let stir 10 minutes) because they are poorly soluble in water. Repeat the treatment after 24 and 48 hours with a 10% water change every day thereby increasing the effective concentration. Remove carbon filters during treatment. Wear plastic gloves to avoid contact with water. Reinstall carbon filters at 72 hours to remove the drugs from the system. Repeat the treatment protocol at 10 days because the cidal effect of these drugs on freshly fertilized eggs or dauerlarvae is not reported.

    Continue testing fish at regular intervals and retreat if necessary.


    Goven, B.A. and Amend D.F. (1982) Mebendazole/trichlorfon combinations: A new anti-helminthic for removing monogenetic trematodes from fish. J. Fish Biol. 20:373-378.

    The Zebrafish Book