Practical Koi & Goldfish Medicine


Koi and goldfish are closely related members of the Family Cyprinidae. These beautiful animals are long-lived and make great pets. Clients may develop strong emotional attachments to their fish, and it is not uncommon for there to be several thousand dollars’ worth of fish in a collection. Because of their enormous popularity this presentation is geared toward koi (Cyprinus carpio), however the information also applies to goldfish (Carassius auratus).

Goldfish (Carassius auratus) routinely live 10-30 years, and over 40 yrs has been recorded. Although some varieties of goldfish resemble koi, adult goldfish are much smaller than adult koi and lack barbels around the mouth.

Koi (Cyprinus carpio) can live 50-70 years, or more. Young koi can grow 10-25 cm/yr, and adults may reach 1 m in length.


Make every effort to avoid damaging the cuticle, removing scales, or tearing fins when handling fish. Minor injuries to the cuticle have the potential to become infected when fish are stressed.

The author prefers not to lift koi out of the water with a net. Instead, he coaxes the fish from the net into a partially submerged plastic garbage bag without lifting the fish out of the water. Then the net is placed beneath the bag for support. The fish can be examined right at the edge of the pond or transferred to a holding tank for closer examination.

Physical Exam

Look closely for skin lesions and external parasites. Assess the mucus level. To accurately visualize the fins, it may help to view them in the water. Assess the corneas for clarity. Look inside the mouth for tumors, ulcers, or other lesions. Gently palpate the abdomen. With the fish in dorsal recumbency it is possible to lift the operculum and assess the color, texture, and mucus level of the gills. Koi will often relax and tolerate minor procedures if restrained in this position, however, for other procedures sedation may be necessary.


Tricaine methane sulfonate (Finquel/MS-222, Argent Labs) 100-200mg/L will anesthetize koi within 3-5 minutes. Use 50-100 mg/L for maintenance. Monitor respiratory rate; opercular movement should always be visible. Move the patient to untreated water for recovery, and release only after it is able to swim on its own.

Sample Collection

Perform a skin scrape using a glass cover slip to gently remove mucus, starting behind the operculum and continuing to the tip of the tail fin. Place the cover slip on a glass slide with a drop of water. A small portion of a fin should be biopsied, and if there are loose scales, one of those should also be examined. To perform a gill biopsy, place the fish in dorsal recumbency, and lift the operculum to expose the gill arches. Using iris or suture scissors, remove a few gill filaments, and place them on a glass slide. Observe biopsy samples for excess mucus, vasculitis, necrosis, parasites, bacteria, and fungi. Samples should be examined immediately for the most accurate results. Venipuncture can be achieved through either a ventral or lateral approach to the caudal vein, using a 3ml syringe with a 22 ga needle, pretreated with sodium heparin or lithium heparin. Culture swabs should be taken from skin ulcers and gill lesions. Bacterial cultures taken from fish should be incubated at lower temperatures 68-77°F (20-25°C) than for other animals.

Common Problems

Poor Water Quality: Ammonia, nitrite, nitrate, and pH should be monitored regularly. Depending upon the individual system, other parameters may need to be monitored and corrected. Poor water quality will cause stress, making fish more susceptible to other diseases. Partial water changes (e.g. 25% per month) will help to prevent the buildup of impurities. In emergency situations, water changes of 50% or more per day may be necessary.

Water quality parameters for koi and gold fish

Measurement Optimum Range Temperature 65-77°F (18-25°C) Oxygen Content 8-12 mg/L pH 7.0-8.0 (stable) Alkalinity 100-250 mg/L Hardness 100-250 mg/L Ammonia 0-0.01 mg/L (unionized) Nitrite 0-0.01 mg/L Nitrate 0-20mg/L Chlorine 0 mg/L (From Saint-Erne N: Advanced Koi Care for Veterinarians and Professional Koi Keepers. Phoenix, AZ, Metagraphix, 2002; with permission) Parasitic Diseases Parasites typically begin causing problems within several months of introducing new fish into a system. They also emerge whenever overcrowding, extreme weather, or similar stress hampers immune function. Fish Lice (Argulus): This external parasite is a dark green, disk-shaped crustacean resembling a small (2-4mm) horseshoe crab. From a distance owners may observe that ‘black spots” have appeared on the fish. They adhere to the fish’s skin, are motile, and can swim. They cause irritation and open pathways for bacteria and fungal infection. Affected fish exhibit “flashing” behavior: quickly swimming to the bottom of the pool and rubbing the side of the body against the floor. Treatment: malathion, lufenuron Anchor Worm ( Lernaea ): Appears as a small ulcer with a 10-40 mm ‘Y’-shaped tag hanging from it. This crustacean parasite is attached proximally by its anchor-like head, with its abdomen and two egg sacs suspended. It is fixed in location and unable to move. The ulcer can become secondarily infected with Aeromonas or Pseudomonas. Gently remove the parasite with forceps coat the attachment site with topical antiseptic. Treatment: malathion, lufenuron Monogenean Trematodes: Gill flukes (Dactylogyrus) and skin flukes (Gyrodactylus) attach with their sharp clawlike ‘haptors’ to the skin, fins, and gills to feed. This creates small lesions which allow for secondary bacterial invasion. Affected fish exhibit exaggerated respiratory movements, clamped fins, and flashing. Microscopically they cause gill hyperplasia resulting in hypoxia. Treatment: malathion, praziquantel, formalin Chilodonella: A motile, heart-shaped, ciliated protozoan that attaches to the gills and skin causing respiratory distress, clamped fins, and excess mucus production. A winter parasite, it reproduces well in cold water. Excess mucus gives affected fish a white or bluish hue. Gill damage by Chilodinella can be severe and result in the rapid loss of many fish. Treatment: salt Ichthyobodo (Costia): A comma-shaped ciliate parasite that primarily affects the gills, inhibiting respiration, and is capable of causing rapid death. Costia may be attached or free swimming. Infection is most common in the spring, but is active in water up to 84°F (29°C). Affected koi will gape, hang near the waterfall or surface. It may also cause reddened fins and excess mucus production. Treatment: salt Ich (Ichthyopthirius): A well known ciliate parasite that appears grossly as punctuate white spots on the skin, fins, and gills. Flashing may be seen. Microscopically, has a classic horseshoe-shaped nucleus and is motile. Causes gill hyperplasia, making gills appear pale and resulting in hypoxia. Embedded trophozoites are resistant to treatment, complicating treatment. Treatment: partial water changes, increased water temperatures, and salt Red Sore Disease (Epistylis, Heteropolaria): Sessile, colonial, ciliated parasite that attaches itself to the body, fins, and gills of fish by a retractable stalk. It is often associated with Aeromonas (the A-E complex). Grossly indistinguishable from fungus, it appears as whitish tufts on affected fish. Affected fish may exhibit flashing. This parasite does not feed on the fish, but on organic debris in the system. Reducing the organic load in the pond is important in preventing Epistylis. Treatment: salt Mitraspora cyprini: This myxosporidian protozoan infects the kidney, causing it to enlarge and fail. Known as “kidney bloater disease”, affected fish may develop dropsy, abdominal swelling, or buoyancy problems. The disease occurs more commonly in goldfish than in koi, and symptoms typically occur during the fall and winter. Bacterial Diseases While some presentations are very suggestive of a particular bacterial infection, an accurate diagnosis requires a wet mount, cytological staining, and a bacterial culture and sensitivity. Hemorrhagic septicemia: Aeromonas is an opportunistic pathogen that is almost always in the water and on fish. Its symptoms include hemorrhages of the gills, skin, base of fins, loss of scales, skin ulcers, exophthalmia (pop eye), abdominal swelling (dropsy), and swim bladder disease. Parasitism, injury, stress, and poor water quality predispose fish to infection. Most Aeromonas are motile Gram negative rods with a single polar flagellum. Furunculosis (Ulcer disease): Caused by atypical Aeromonas salmonicida, which is a short, Gram negative, non-motile rod. Fish are most susceptible in the spring when they are stressed from over wintering, and immune function has not yet reached its peak. This bacteria causes erythema and skin ulcers when water temperatures range between 40°F (4.4°C) and 68°F (20°C). Ulcers can be severe, and hemorrhage of internal organs also occurs. Infected koi should be isolated so as to reduce the spread of infection. Fin and Gill Rot: Pseudomonas is a short, motile Gram negative rod. Infection is usually associated with fin and tail erosion, but may also be associated with hemorrhage and ulceration of the skin or gills. While fin and tail erosion is typical of Pseudomonas, an accurate diagnosis requires a wet mount, cytological staining, and a bacterial culture. Traumatic injuries often become secondarily infected. Pseudomonas can be differentiated from non-motile Aeromonas salmonicida by a wet mount or culture. Columnaris disease (Cotton Mouth): Flavobacterium (Flexibacter, Cytophagia) columnaris is a Gram negative opportunist that attacks weakened or injured fish. Flexibacter usually causes lesions on the lips, fins, and dorsal aspect of the caudal peduncle. It can also affect the gills. Cottony white lesions grossly resemble fungal infection. Wet mounts reveal long, thin, motile rods that aggregate in a “haystack” formation. Infections occur in cold and warm water however it is most pathogenic in water above 64°F (18°C). Poor water quality and accumulated detritus may promote infections. Treatment: Use injectible antibiotics for severe infections. If fish are stable and still eating, a medicated diet can be used. Medicating the water is the last option, because antibiotics interfere with the biological filtration of the system. Salt should be added in order to reduce osmotic stress on the collection. Treat ulcerations with topical disinfectants and antibiotic preparations (see therapeutics, below) Mycobacteriosis Mycobacteria are very small, Gram positive, acid fast coccobaccilli that cause acute to chronic peritoneal infection. Signs include wasting, lethargy, anorexia, skin lesions, exophthalmia, and ascites. At necropsy, granulomas may be observed in the liver, kidney, and other organs. Mycobacteriosis is spread by cannibalism. The prognosis is poor and treatment is usually unrewarding. Zoonotic potential exists: cuts and scrapes on a person’s hands can become infected. The resulting localized skin infection is known as “fish handler’s disease”. Fungal Disease Saprolegnia occurs on tissues that have previously been damaged by trauma or disease. Fungal hyphae assist in the destruction of adjacent healthy tissue. Infections appear grossly as white cottony tufts on the skin or gills. Saprolegnia is most abundant in ponds where large amounts of organic debris (mulm) has accumulated, so water changes and detritus removal are helpful in the management of an infection. Treatment: formalin, malachite green Viral Disease Carp Pox (Cyprinid Herpesvirus 1): Results in waxy, smooth, raised milky-white or pinkish plaques (epidermal hyperplasia) on the surface of the body and fins. Lesions are benign, eventually slough, and sometimes cause scarring. Carp pox occurs more often in cold water and in overcrowded ponds. Isolate affected koi. Treatment: surgical removal, topical disinfectants, Acyclovir (no treatment is completely effective) Spring Viremia of Carp (Rhabdovirus carpio): Produces a wide variety of signs including ascites (dropsy), exophthalmia (pop eye), vent inflammation, incoordination, weak respiration, and hemorrhages of the gills, skin, and swim bladder. Infections are more common in cool water. Has been linked to Aeromonas infection, and may be transmitted by parasites. Spring Viremia is a reportable foreign animal disease. Treatment: warming the water to 68°F (20°C) or greater, UV filter, formalin Koi Herpes Virus: A highly contagious herpes viral infection affecting koi and common carp. KHV infection may produce severe gills lesions, and affected populations may experience 80-100% mortality. Onset is very rapid. Fish are most susceptible at water temperature of 72-81°F (22-27°C). Fry are more susceptible than adults. Recovered individuals are potential carriers for life. There is no known treatment for KHV. Swollen abdomen Abdominal swelling may be the result of ascites (also see Dropsy, below), hepatic lipidosis, neoplasia, organomegaly, infection, abscess, egg or fecal impaction. Fatty liver degeneration is common in koi which are fed a diet high in fat or containing low-quality protein. Neoplasia, especially of the gonads, is common, as are cystic kidneys. Organs may swell as a result of infection, and abscesses can grow large enough that they impact other organs. Egg or fecal impaction may be the result of abscesses, infection, neoplasia, or other causes. Radiology and/or surgery may be indicated. Swelling of the abdomen can become very severe before it impairs normal activity in fish. Dropsy Ascites (dropsy) is particularly common in koi and goldfish as a result of kidney, liver or other organ failure. Fluid accumulation in the peritoneal cavity presents as marked abdominal swelling, so that scales protrude from the side of the body in a “pinecone” fashion. Aeromonas, Mycobacterium, and Rhabdovirus and other infections can result in dropsy. The prognosis is generally poor, but affected fish should receive antibiotics and optimal supportive care. Dropsy may resolve only to return following a stressful event. Neoplasia Various tumors have been reported in koi and goldfish. Skin tumors, fin tumors, and intra-abdominal tumors associated with the gonads are common. Abdominal tumors may be evidenced by abdominal distension, asymmetrical swelling, or buoyancy disorders. Organ dysfunction can occur as a result of pressure from large abdominal masses. Hepatic neoplasia is known to occur as a result of feeding moldy fish foods containing the mycotoxins produced by Aspergillus flavus. Buoyancy Problems The swim bladder of koi and goldfish has two chambers connected by an isthmus. The caudal chamber is connected to the esophagus by the pneumatic duct, through which air can pass to and from the swim bladder. Infections, abscesses, granulomas, trauma, neoplasia, egg retention, and torsion of the swim bladder can disrupt the duct or displace the swim bladder, affecting buoyancy. In some goldfish, gas accumulation in the GI tract can cause similar problems. Trauma Fish occasionally injure themselves in and around pond rocks and furnishings. Birds of prey and wading birds are capable of inflicting severe wounds. Secondary bacterial and fungal infections are likely. Treat lesions with topical antiseptics such as povidone-iodine solution, merthiolate, and silver sulfadiazine cream, and use systemic antibiotics where appropriate. Scoliosis Electrocution by lightning or faulty electrical equipment (e.g. submersible pumps) is documented to cause spinal trauma in fish. Affected koi display kinked backs, swim erratically, and display buoyancy problems. Scoliosis can result from vitamin C or tryptophan deficiency. Other causes include spinal trauma, muscle damage, organophosphate toxicity, and arsenic exposure (e.g. pressure treated lumber). Nutritional Diseases Vitamin C deficiency: Ascorbic acid is necessary for normal collagen metabolism. Deficiency results in poor bone and cartilage formation, leading to curvature of the spine. To reduce the decay of vitamin C in processed koi diets, keep food in airtight containers and store in cool, dry areas. Diets containing a stabilized form of vitamin C have a longer shelf life. Purchase smaller quantities of feed in order to increase turnover. Supplemental vitamin C can be provided by offering oranges, spinach, turnip greens, broccoli, and other high-ascorbate vegetables for koi and gold fish to graze on. Fatty liver: Koi raised on a diet that is high in fat, or contains corn and other low quality, plant-based proteins (e.g. catfish chow) may develop hepatic lipidosis. Prevent fatty liver disease by providing seafood and animal-based protein in the diet. Therapeutics Salt (sodium chloride) Before introducing new fish into an established pond, they should be dipped in a 2% salt solution for 10-20 minutes. This will kill external protozoans and trematodes. Discontinue if the fish becomes disoriented and unable to stay upright. Adding salt to the pond water at a rate of 2-3lbs/100gal (0.2-0.3%) will kill ciliate parasites and reduce osmotic stress on the fish. Up to 5lbs/100 gal (0.5%) may be used for this purpose, although above 0.5lbs/100gal (0.05) may damage pond plants. Antibiotics Antibiotic use in the fish hobby is wide spread and resistant bacterial infections are common, therefore culture and sensitivity is important when selecting an antibiotic. Antibiotic treatment for koi is generally continued for 14-21 days. Tube feeding Anorectic koi can often be stimulated to eat by administering softened food by syringe and tube. Medications can be mixed with food and given in this manner. Small volumes are used (1-5 cc depending on size of fish) because koi lack a stomach. Medicated Diets Antibiotic medicated feeds have the advantage of treating many fish without handling, but are of little value if fish are not eating. Commercial medicated diets are available, as is a gelatinized diet for customizing a formulation. The author has had good success with a homemade gelatinized recipe. Gelatinized Diet recipe

  1. Weigh 125 grams of flake or pelleted fish food and place it into a blender. Add 250 ml of clean water and mix well
  2. To this slurry add one can of sardines (with oil) and a half jar of baby food spinach (or use fresh spinach). Blend well.
  3. Add any medications at this time
  4. In a separate pan heat 250 ml of clean water to near boiling. Add 2 packets (7 grams each) of gelatin and stir well. Allow the gelatin to cool for about 10 minutes, then add it to the food/medication mixture and blend well.
  5. Pour mixture into ice cube trays or resealable plastic bags to make feeding easier. Place in refrigerator.
  6. The food should set in several hours and can be fed at that time.

Recipe will yield approximately 750 grams of food. Recipe may be halved or doubled depending on amount of food desired. Final product should be stored cold or frozen. Medicated diet will keep for about 7 days in the refrigerator. References

  1. Saint-Erne N: Advanced Koi Care for Veterinarians and Professional Koi Keepers. Phoenix, AZ, Metagraphix, 2002.
  2. Johnson EL: Koi Health and Disease. Athens, GA, Reade Printers, 1997.
  3. Gratzek JB, Matthews JR (eds): Aquariology: the science of fish health management. Blacksburg, VA, Tetra Press, 1992.
  4. Carpenter JW, Mashima TY, Rupiper DJ: Exotic Animal Formulary. Philadelphia, WB Saunders Co, 2001, pp 1-21.
  5. Lewbart GA: Pet Fish Therapeutics. The Proceedings of the Western Veterinary Conference, 2002 (online at VIN)
  6. Lewbart GA: Surgical Techniques in the Koi Patient. ExoticDVM 3.3:43-47.
  7. Stoskopf MK (ed): Fish Medicine. Philadelphia, WB Saunders Co, 1993.
  8. Kizer A, Novo RE: Ovariectomy in Rainbow Trout for Prevention of Postovulatory Stasis. ExoticDVM 5.1:27-30.