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Management of Fish

By Barbara D. Petty, DVM, North Florida Aquatic Veterinary Services ; Ruth Francis-Floyd, DVM, MS, DACZM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

Many management principles applied to other species are equally relevant for fish, including taking a good history, considering appropriate diagnostic techniques, and being cognizant of therapeutic options, water quality issues, and quarantine and biosecurity procedures.


Fish are poikilothermic, and all physiologic processes are greatly influenced by water temperature. In freshwater, the internal tissues of fish are hyperosmotic to the environment, whereas in saltwater they are hypoosmotic. Surface injuries to the skin make osmoregulation more difficult and may be of serious consequence because of the loss of fluid balance and circulatory collapse.

The structure of the fish kidney varies with the species; generally it is divided into an anterior “head” kidney, which is usually located anterior to the gas bladder, and a posterior “caudal” kidney, which is retroperitoneal and ventral to the vertebral column. Hematopoietic, renal, and endocrine tissues are found in the kidney, with hematopoietic tissue located cranially and excretory tissue located caudally. Divalent ions are excreted principally via the kidney, and monovalent ions and nitrogenous excretions via the gills. Accordingly, lesions of the kidney and gills may seriously interfere with respiration, excretion, and fluid balance.

The gas bladder (also known as the swim or air bladder) in bony fish, which originates as an appendage of the foregut, regulates body buoyancy and may also be used for sound production. Physostomous fish have an open connection between the swim bladder and the GI tract, whereas physoclistous fish do not. Although a single chamber in most fish, the gas bladder consists of dual chambers in cyprinids and three chambers in cod and suckers belonging to the genus Moxostoma.

A sensory lateral line system along the sides of the body and head receives stimuli from the aquatic environment and mediates adaptive responses through the CNS.

Fish depend on increases in environmental temperature for efficient antibody production during infections (or after vaccinations), when most pathogens are replicating at a more rapid rate. The optimal temperature for antibody production varies with the species of fish (tropical, temperate, or coldwater). Extremes in environmental temperature (above or below that of the natural habitat) inhibit antibody production. Like T lymphocytes in other vertebrates, those in fish are responsible for cell-mediated immunity. Immunity is not as age dependent in fish as it is in other animals; young fish are usually immunocompetent and can be vaccinated successfully. Antibodies are found in the mucus of the fish skin and GI tract.

Although vaccination of fish against specific diseases has been economically important in preventing losses, there is a need for improved methodology. Advances include increased use of autogenous vaccines; several companies will work with veterinarians and their clients to develop custom vaccines for specific situations. A few vaccines (eg, for Aeromonas salmonicida) are available or in development for pet fish, particularly koi.


As with all species, a good history is critical to establish a diagnosis. Questions of particular interest for fish cases include the number of animals affected, whether one species or multiple, the chronicity of the problem, and a thorough description of animal housing and care, including the volume and design of the system (see Aquatic Systems), number and size of animals stocked, species, new additions, use of quarantine, and previous medications. Owners can be asked to bring fish and water samples into the clinic, or the practitioner may wish to visit the site. Site visits allow the system to be more accurately evaluated and the behavior of fish readily observed. If fish are brought to the clinic, the owner should provide an animal showing the signs of concern. A live animal can be transported in a cooler with a battery-powered aerator. A separate water sample should be provided in a plastic bag and transported on ice. A minimum of 1 quart of tank water should be requested for analysis. If desired for recovery after anesthetizing the animal, a larger volume of tank water may be requested. Recently deceased specimens have diagnostic value and may be submitted to the veterinary clinic, or the owner may be directed to submit these to a laboratory experienced in fish necropsy and diagnostic testing (see below). Water samples should be submitted with necropsy specimens.

Necropsy and Diagnostic Techniques

Although the same principles are used in necropsy of fish as in other animals, great emphasis is placed on an accurate and thorough history, premortem signs, fresh necropsy material, and direct microscopic examination of fresh tissue smears and squash preparations. Fish decompose quickly, and many saprophytic microorganisms reproduce rapidly in the decaying tissues, which complicates isolation of pathogens unless samples are collected immediately after death. A general fish necropsy may include blood collection (premortem); biopsy of gill, skin, and fin tissues; bacterial or viral culture of internal organs; and histology. A veterinary clinic or diagnostic facility familiar with fish necropsy protocols and aquatic microbiology should be used. Whenever possible, live fish should be submitted. If the fish has just died, the eyes should be clear and the gills normal in coloration and texture. There should be no “dead fish” smell; however, some moribund fishes may have a strong odor. Freshly dead fish should be placed in a sturdy plastic bag and submitted on ice. A water sample should always be submitted with the fish. An animal that has died and been placed in a freezer has limited diagnostic value, but freshly dead frozen fish may be useful for bacteriology, virology, or toxicology testing.

Fresh tissue samples of gill filaments, skin mucus, and fins should be collected, prepared as a wet mount, and examined under a light microscope at 40×, 100×, and 400×. Fresh water should be used to prepare wet mounts of external tissues from freshwater fish, and saltwater should be used to prepare wet mounts from marine fish. If uncertain, water from the tank or from the submitted water sample should be used. Ensuring that the salinity used to prepare mounts is similar to the salinity present in the environment should allow organisms to remain viable long enough for identification. Distilled water should not be used for tissue samples. Tissue should be examined for morphology and for the presence of parasites, bacteria, or fungal elements. Microscopic examination of internal organs is also recommended if the fish has been euthanized. Unstained sections of stomach and intestine should be examined for presence of parasites, and the lower intestine should be examined for flagellates. Unstained sections of spleen, anterior and posterior kidneys, and liver should be examined for presence of parasites, granulomas, or other anomalies. Wet-mount examination of fish tissues is crucial for diagnosis of most parasites.

Blood can be collected from a number of sites, and hematologic parameters measured. In fish >25–100 g, depending on species, the caudal vessels of the caudal peduncle, the duct of Cuvier (common cardinal vein), and the dorsal and ventral aortas are easily accessible. For smaller specimens that are to be euthanized, blood can be collected in a hematocrit tube immediately after euthanasia by severing the caudal peduncle and exposing the caudal vessels. Use of hematology and serum chemistry is limited, because normal values for many fish species are not readily available; however, the information may still be clinically useful. Lithium heparin is the anticoagulant of choice for most fish species, although EDTA is preferred for ictalurid catfishes, and plasma may be used for biochemistry tests. Serology may be diagnostic in certain cases (eg, heavy metal toxicity). Whole blood (1–2 drops) can be incubated in brain heart infusion broth at room temperature on an electric rotator. If cloudiness indicative of bacterial growth develops, a loop of the blood-broth mix can be used to attempt primary isolation of a systemic bacterial pathogen.

Fish should be euthanized and opened under sterile conditions. A sterile swab of posterior kidney, or other organ of interest, may be shipped to a laboratory in transport media, but primary isolation directly onto an enriched media (eg, tryptic soya agar enriched with 5% sheep blood) is preferable. Although blood agar supplemented with salt is helpful for marine fish, it is not necessary if an enriched blood agar is used. Ordal’s or similar cytophaga media should be available for isolation of myxobacteria (slime bacteria, including Flavobacterium columnare). Sabouraud agar is an excellent all-purpose media for isolation of fungal agents. Lowenstein-Jensen or Middlebrook media is recommended for isolation of Mycobacterium spp. Mueller-Hinton is the media of choice for susceptibility testing of most common bacteria isolated from fish. If abscesses or other obvious anomalies are visible, those sites should also be cultured. In general, bacterial or fungal cultures taken from fish tissue should be incubated at room temperature (25°C). Some agents of concern will not grow at all at 37°C, the standard temperature for incubation of cultures taken from mammals. Agents of zoonotic concern, such as Mycobacterium, can be dual incubated at both 25°C and 37°C. An acid-fast stain should be available for bench-top staining of granulomatous material which, when positive, is strongly suggestive of Mycobacterium. If fish are seen spinning or showing other behavioral indications of neurologic disease before death, brain cultures are indicated.

If viral disease is suspected, appropriate tissues may be collected. Specimens should include both fresh tissues placed in reagent ethanol and frozen tissues. Several viral diseases are of regulatory concern to veterinarians practicing fish medicine in the USA (see Table: Fish Diseases of Regulatory Concern in the USA).

Fish Diseases of Regulatory Concern in the USA


Causative Agent

Susceptible Species

Clinical Signs and Pathology

Temperature Range

Status in USA

Viral hemorrhagic septicemia (Egtved disease)


Family: Rhabdoviridae

Primary: salmonids (Oncorhynchus spp), turbo, herring and spat (Clupea spp), Japanese flounder

Secondary: grayling, whitefish, pike, Atlantic and Pacific cod, haddock, many freshwater, marine, and estuarine species

Acute form: nonspecific hemorrhaging (eyes, fins, skin), darkening, exophthalmia, ascites

Chronic form: few signs

Neurologic form: spinning/flashing

Gross: enlarged spleen, ascites, necrotic kidney

Histologic: focal necrosis of kidney liver, spleen; hemorrhage in muscle

9°–12°C (48°–54°F) optimal

Present in wild populations, sporadic, limited distribution; endemic in Pacific Northwest and Alaska (wild salmonids, haddock, and cod); emerging disease in Great Lakes region

Infectious hematopoietic necrosis


Family: Rhabdoviridae

Primary: cultured salmonids (Oncorhynchus spp); lake trout and char (Salvelinus spp) are resistant

Rapidly increasing mortality (fish <1 yr), lethargic but sporadic bursts of rapid swimming occur, protruding vent, fecal casts, exophthalmic, pale gills, darkening, abdominal distention/ascitic fluid (possibly bloody)

10°–12°C (50°–54°F) optimal; rare >15°C (59°F)

Present in western USA, sporadic, limited distribution; endemic in Pacific Northwest and Alaska (wild salmonids); also present in parts of Europe and Asia

Spring viremia of carp


Family: Rhabdoviridae

Primary: carp (including koi, goldfish), sheatfish (European catfish), orfe, tench

Nonspecific: darkening, exophthalmic, pale gills, distended abdomen, ascites, hemorrhage (gills, skin, eye), petechiae in organs (including swim bladder), protruding vent with thick mucoid fecal cast

Coinfection with Aeromonas or other bacteria common

12°–22°C (54°–72°F)

USA is free (last occurred in captive fish in 2004, wild fish in 2007); occurs in eastern Europe, Russia, China, and Middle East

Epizootic hematopoietic necrosis


Family: Iridoviridae

Primary: redfin perch

Secondary: rainbow trout

(wild and farmed fish)

Acute and high mortality of redfin perch; darkening, ataxia, lethargy, hemorrhage around nares; morbidity and mortality of rainbow trout less severe; Histologic: necrosis, renal hematopoietic tissues

Redfin perch: >12°C (54°F)

Rainbow trout: 11°–17°C (52°–63°F)

Experimental: 8°–21°C (46°–70°F)

Has never occurred in USA; endemic in Australia

Endemic in Australia

Red sea bream iridoviral disease



Red sea bream, many

other estuarine species, other marine species

Severe anemia, lethargic, pale gills, enlarged spleen

Has never occurred in USA; occurs in Japan and Taiwan

Infection with HPR-deleted or HPR0

Infectious salmon anemia


Family: Orthomyxoviridae

Atlantic salmon

Brown trout, sea trout, rainbow trout

Pale gills, severe anemia (PCV <10%), swollen liver (black/brown color), ascites, petechiae of viscera, mesenteric fat, swim bladder

In vitro:

Maximum replication 15°C (59°F)

No replication 25°C (77°F)

Present in northeast USA, sporadic, limited distribution; endemic in Maine, New Brunswick, Scotland, and Norway

Endemic in Maine, New Brunswick, Scotland, and Norway

Infection with salmonid alphavirus (pancreas disease or sleeping disease)


Family: Togaviridae

Atlantic salmon, rainbow trout, brown trout

Necrosis of exocrine pancreas, heart and skeletal muscle changes

12°–15°C (54°–59°F)

Has not occurred in USA; detected in Ireland, UK, France, Germany, Italy, Spain, Switzerland, Poland, and Norway

Koi herpesvirus


Cyprinid herpesvirus-3

Common carp and hybrids, including koi and ghost carp

Severe necrosis gill tissue

22°–25.5°C (72°–78°F) optimala

Present in USA, sporadic, widely distributed

Epizootic ulcerative syndrome (mycotic granulomatosis)

Aphanomyces invadans

Atlantic menhaden, striped mullet, many other freshwater and estuarine species; snakeheads, barbs (Puntias spp) sensitive; gouramis, goldfish and other ornamentals susceptible; tilapia resistant

Necrotizing deep ulcers (penetrate body wall), granulomatous tissue response; deep ulcers with red centers, white rims; invasive nonseptate hyphae (culture possible but difficult)

<25°C (77°F)

(reduced salinity also contributes in brackish systems)

Present in USA, sporadic, limited distribution

Gyrodactylus (Gyrodactylus salaris only)


Gyrodactylus salaris only

Atlantic salmon, rainbow trout, brook trout, North American lake trout, brown trout, grayling arctic char

Has never occurred in USA

a Mortalities stop at >30°C (86°F), but survivors remain carriers.

Tissue sections no larger than 1 cm3 should be placed in 10% neutral buffered formalin for histopathology. After 24 hr of fixation, they should be removed and placed in reagent alcohol in case molecular diagnostic techniques will be required.

Therapeutic Considerations

Therapy for pet and ornamental fish is often based on environmental management followed by the use of targeted therapy to control specific pathogens. Use of prophylactic medication in the absence of diagnostic testing is strongly discouraged and may contribute to resistant bacterial infection and other complications.

Drug therapy can be provided via several routes of administration, including exposure by bath (adding medication to water), medicated feed, injection, or topical administration. Generally speaking, bath and topical treatments are most useful for external infections, whereas medicated food and injection are most appropriate for internal infections.

Using a bath treatment requires accurate measurement of the volume of water to be treated. Volume of a rectangular tank is calculated by multiplying measurements of the length, width, and depth. Volume of a circular tank is calculated by multiplying 3.14 by the radius squared by the depth. To calculate directly into liters, measurements should be in cm and multiplied by 0.001. If the measurements are made in feet, the result will be in cubic feet; cubic feet can be converted to gallons by multiplying by 7.481. If the container is oddly shaped, volume may be calculated mathematically, but it may be easier to purchase a flow meter to measure the volume required to fill the tank. Alternatively, the volume of inflowing water per minute can be measured by determining how long it takes to fill a 1-L graduated cylinder (or 5-gal. bucket). Using that information, determining how long it takes to fill a tank or ornamental pond can provide a fairly accurate assessment of volume.

Some medicated feeds can be purchased commercially for aquaculture or pet fish. Custom-made medicated feeds can be prepared for use in ornamental fish. Flake, pellet, or gel diets can be used as a base for medicated food for pet fish. Cooking oil spray is an effective binder for use with pelleted or flake foods for ornamental fish. The addition of medication to commercially available gel diets is easily done as the gel cools. In general, medication should not be added when the gelatin is hot, because some medications, particularly oxytetracycline, are heat labile.

Injections can be given either IM or intracoelomic (IC). IM injections are given in the epaxial muscles, lateral to the dorsal fin. Injection of some drugs can cause muscle necrosis, so it is important to alternate injection sites and to limit injections to every 3 days unless required more often. To administer IC injections, fish should be placed head downward in dorsal recumbency to move internal organs away from the injection site, which should be anterior to the anus, just off the ventral midline. Topical treatments, usually in ointment form, should be applied directly to the external lesion(s). The fish can be manually restrained, usually without removing the entire animal from the water. The area to which the treatment is applied should be held out of the water for several seconds (<1 min) to allow some absorption before returning the fish to the water. Repeated applications may be necessary.

Sunburn can occur in surface-swimming fish or can be induced (even in bottom-dwelling species) by feeding photodynamic drugs such as phenothiazine, although ultraviolet light penetrates water poorly. Affected fish will have visible lesions along the dorsal surface. Providing shade solves the problem.

FDA-approved Drugs and Regulatory Concerns:

FDA-approved therapeutic options in fish are limited; however, there has been significant progress in approvals in the past few years, and this trend is expected to continue. The FDA Web site ( is the best resource for current information on the status of drugs and chemicals. In many cases, therapeutic management of fish other than catfish or salmonids requires extra-label use of drugs. A new mechanism called indexing is being developed by the FDA to allow for legal use of nonapproved drugs in ornamental fish. The following information is intended for use in pet fish medicine.


Antibiotics can be delivered to pet fish through any of the treatment routes listed; however, medicated food is most common and usually most effective. Common antibiotics used in pet or ornamental fish include oxytetracycline, potentiated sulfa drugs, and enrofloxacin (in koi and exhibit fish). For approved dosages and withdrawal times, see Table: FDA-Approved Drugs for Aquaculture Use in the USA (2014). Oxytetracycline (Terramycin-200®) can be fed at a dosage of 55–83 mg/kg/day for 10 days to control many gram-negative bacterial infections, including columnaris disease. Because oxytetracycline has been on the market for many years, there may be significant resistance to it in some bacterial isolates. Romet-30® (sulfadimethoxine and ormetoprim) also is effective against many gram-negative organisms, with less resistance. Palatability can be a problem when feeding medicated food to sick fish, which may have a poor appetite.

FDA-Approved Drugs for Aquaculture Use in the USA (2014)




Dosage Regimen



Formalin (Parasite-S®, Formalin-F™, Formacide-B)

All finfish

Control external protozoa (Chilodonella, Ichthyobodo, Epistylis, Ichthyophthirius, Ambiphyra, Trichodina spp), Tetrahymena spp and monogeneans (Cleidodiscus, Dactylogyrus, Gyrodactylus spp)

Tanks and raceways: salmon and trout: above 50°F: up to 170 μL/L for up to 1 hr; below 50°F: up to 250 μL/L for up to 1 hr. All other finfish: up to 250 μL/L for up to 1 hr

Earthen ponds: 15–25 μL/L indefinitely

Drug must not be subjected to temperature <40°F. Do not apply to ponds when water is warmer than 80°F, there is a heavy phytoplankton bloom, or dissolved oxygen is <5 mg/L. Ponds may be re-treated in 5–10 days if needed. Do not treat ponds containing striped bass. Test on a small number from each lot to check for any unusual sensitivity to formalin before proceeding.

All finfish eggs

Control fungi of the family Saprolegniaceae

All finfish eggs: 1,000–2,000 ppm for 15 min; Acipenseriformes: up to 1,500 ppm for 15 min

Preliminary bioassay should be conducted to determine species sensitivity.

Penaeid shrimp

Control protozoan parasites (Bodo, Epistylis, and Zoothamnium spp )

Tanks and raceways: 50–100 μL/L for up to 4 hr daily

Earthen ponds: 25 μL/L as single treatment

Drug must not be subjected to temperature <40°F. Do not apply to ponds when water is warmer than 80°F, when there is a heavy phytoplankton bloom, or when dissolved oxygen is <5 mg/L. Ponds may be re-treated in 5–10 days if needed.

Salmon, trout, and esocid eggs

Control fungi of the family saprolegniaceae

1,000–2,000 ppm for 15 min

Preliminary bioassay should be conducted to determine species sensitivity.

Hydrogen peroxide (35% PEROX-AID®)

Freshwater-reared finfish eggs

Control mortality due to saprolegniasis

Coldwater and coolwater: 500–1,000 mg/L for 15 min in a continuous flow system daily on consecutive or alternate days until hatch

Warmwater: 750–1,000 mg/L for 15 min in a continuous flow system daily on consecutive or alternative days until hatch

Initial bioassay on a small number is recommended before treating entire group.

Freshwater-reared salmonids

Control mortality due to bacterial gill disease (Flavobacterium branchiophilum)

100 mg/L (30 min) or 50–100 mg/L (60 min) daily on alternate days for 3 treatments

Initial bioassay on a small number is recommended before treating entire group.

Freshwater-reared coolwater finfish and channel catfish

Control mortality due to external columnaris disease (Flavobacterium columnare)

Fingerling and adults (except northern pike and paddlefish): 50–75 mg/L (60 min) daily on alternate days for 3 treatments

Fry (except northern pike, pallid sturgeon, and paddlefish): 50 mg/L (60 min) daily on alternate days for 3 treatments

Use with caution on walleye.

Initial bioassay on a small number is recommended before treating entire group.

Oxytetracycline hydrochloride (OxyMarine, Oxytetracycline HCI Soluble Powder-343®, Terramycin-343 Soluble Powder, PENNOX 343, TETROXY Aquatic)

Finfish fry and fingerlings

Mark skeletal tissues, most often otoliths, of finfish fry and fingerlings for identification purposes

200–700 mg oxytetracycline hydrochloride (buffered) per liter of water for 2–6 hr

Tricaine methanesulfonate (Tricaine-S®)

Fish, amphibians, and other aquatic poikilotherms

Temporary immobilization

15–330 mg/L (fish)

1:1,000 to 1:20,000 (other poikilotherms)

Powder is added to water; concentration depends on desired degree of anesthesia, species, size, water temperature and softness, stage of development.

Preliminary tests of solution should be made with a few fish; 21-day withdrawal time (fish); laboratory or hatchery use only in other poikilotherms; water temperature >50°F (10°C)


Chorionic gonadotropin (Chorulon®)

Male and female brood finfish

Help improve spawning function

50–510 IU/lb males; 67–1,816 IU/lb females

IM injection up to 3 doses; total dose not to exceed 25,000 IU in fish intended for human consumption; restricted to use by or on the order of a licensed veterinarian

Medicated Article/Feed

Florfenicol (Aquaflor®)

Freshwater-reared finfish

Control of mortality due to enteric septicemia of catfish associated with Edwardsiella ictaluri, streptococcal septicemia in freshwater-reared warmwater finfish, columnaris disease in freshwater-reared finfish

10 mg/kg/day for 10 consecutive days

Veterinary Feed Directive drug; 12-day withdrawal time

Oxytetracycline dihydrate (Terramycin-200® for Fish)

Pacific salmon

Mark skeletal tissue, most often otoliths, of finfish fry and fingerlings for identification purposes

250 mg/kg/day for 4 days

Salmon <30 g; in feed as sole ration; 7-day withdrawal time

Freshwater-reared salmonids

Bacterial coldwater disease (Flavobacterium psychrophilum)

2.5–3.75 g/100 lb/day for 10 days

In mixed ration; water temperature not <48.2°F; 21-day withdrawal time

Scaled warm freshwater-reared finfish

Control bacterial hemorrhagic septicemia (Aeromonas liquefaciens) and pseudomonas disease (Pseudomonas)

Cool freshwater-reared finfish

Control bacterial hemorrhagic septicemia (Aeromonas liquefaciens, furunculosis (Aeromonas salmonicida), and pseudomonas disease (Pseudomonas)

2.5–3.75 g/100 lb/day for 10 days

In mixed ration; water temperature not <62°F; 21-day withdrawal time


Control gaffkemia (Aeroccocus viridans)

1 g/lb medicated feed for 5 days

In feed as sole ration; 30-day withdrawal time

All freshwater-reared finfish

Control of columnaris disease

Sulfadimethoxine-ormetoprim (Romet-30®)


Control furunculosis (Aeromonas salmonicida)

50 mg/kg/day for 5 days

In feed; 42-day withdrawal time


Control enteric septicemia (Edwardsiella ictaluri)

50 mg/kg/day for 5 days

In feed; 3-day withdrawal time


Rainbow, brook, and brown trout

Control furunculosis (Aeromonas salmonicida)

10 g/100 lb/day for up to 14 days

In feed; 21-day withdrawal time; not currently available

Note: This is an abbreviated summary. For complete labeling, see the package insert. Approval applies only to the specific drug that is the subject of a new animal drug application (NADA); active ingredients from other sources (eg, a bulk drug from a chemical company or similar compounds made by companies other than those specified in the NADA) are not approved new animal drugs. Approval applies only to use of the drug for the indications and manner specified on the label.

Florfenicol is available as Aquaflor® for use in medicated feed (see Aquaculture).

Erythromycin is not FDA approved for use in fish but is an excellent antibiotic for fish infected with gram-positive bacteria, particularly Streptococcus. Erythromycin can be fed at a dosage of 100 mg/kg/day, for 14 days. Palatability may be a concern with erythromycin-medicated feed. Erythromycin has been used in management of bacterial kidney disease of salmonids and streptococcal infections in food and nonfood species. FDA permission is required for use in food animals.

Delivery of antibiotics in a bath treatment is not generally recommended because of unknown or limited efficacy and damaging environmental effects (ie, antibiotics tend to kill the biofilter). Oxytetracycline (100–400 mg/L for 1 hr, daily for 10 days) has some efficacy when delivered in a bath. Oxytetracycline in a bath treatment is chelated by hard water and is therefore ineffective in marine systems. Enrofloxacin has been used as a bath at 2.5–5 mg/L for 5 hr, daily for 7 days. Water changes are recommended after the 5-hr contact time, and the drug may be chelated by hard water. Kanamycin has also been used as a bath treatment at dosages of 50–100 mg/L for 5 hr, repeated every 3 days for three treatments, with water changes recommended after the 5-hr contact time. Nephrotoxicity is a concern in fish treated with aminoglycosides. Many other options are discussed in the literature.

Injection is the most effective way to control the amount of antibiotic delivered to a fish. Enrofloxacin can be delivered IC at a dosage of 5–10 mg/kg, and the higher dosage can be repeated every third day, minimizing handling. Three treatments are generally recommended. When using enrofloxacin, the less concentrated dose (22.7 mg/mL) is recommended, even for use in large fish, because of adverse tissue reactions with the concentrated dose. If injection volume is excessive, more than one injection site can be used. Other injectable antibiotics include amikacin (5 mg/kg, IM, every 3 days for a total of three treatments). As with other aminoglycosides, nephrotoxicity may be a concern. Erythromycin (10 mg/kg/day, IM, for 3 days) can be used to treat gram-positive infections in large fish.

Use of topical ointments containing antibiotics can be practical in pet fish. Some seem to absorb fairly quickly, but if treating a substantial wound, it may be appropriate to cover it with a waterproof compound. Frequent application of antibiotic ointment (eg, twice a day) can work well to treat superficial ulcers in pet fish.


Several brands of formalin (37% formaldehyde in aqueous solution, not 10% formalin used for fixation of tissues for histology) are FDA approved as parasiticides for use in finfish and penaeid (saltwater) shrimp (see Table: FDA-Approved Drugs for Aquaculture Use in the USA (2014)). These products are usually used as a prolonged bath at concentrations of 15–25 mg/L. Vigorous aeration during formalin treatment is essential. A concentration of 25 mg/L is equal to 2 drops/gal. or 1 mL/10 gal. (useful for delivering formalin to aquarium fish).

Salt is categorized as of “low regulatory concern” by the FDA and has many uses in fish medicine, including destruction of single-celled protozoans and management of osmoregulation. Salinity is typically measured as parts per thousand (ppt) or g/L (1 ppt = 1 g/L). Seawater is typically 32–37 ppt salt. By increasing or decreasing the amount of salt to which a freshwater or marine fish is exposed, osmoregulatory stress can be minimized and many parasites eliminated. For freshwater fish, a 30 ppt dip for 0.5–10 min, depending on species, is an effective ectoparasiticide and is strongly recommended when moving fish. When fish show signs of distress, commonly manifested by rolling on their side, they should be removed from the bath. The use of salt is a quick and effective way to minimize the introduction of some protistans into a system with new fish. A solution of 5–10 ppt salt is recommended for transportation of freshwater fish; most species will tolerate this concentration for several hours or days. Permanently increasing salinity to 2–3 ppt in a freshwater system can minimize some parasitic protistans. Salt is not generally practical for use in production ponds, except for control or prevention of nitrite toxicity, because of their large volume, but it can be used in ornamental ponds that are not more than several thousand gal. Less information is available on lowering salinity for marine fish; however, freshwater dips adjusted for temperature and pH are often recommended when moving animals. Lowering salinity to 16–18 ppt can be very useful when treating some parasitic diseases, particularly Cryptocaryon.

In saltwater systems, copper is sometimes applied in a chelated form, because it stays in concentration longer. Chelated compounds may be difficult to use safely and require careful monitoring. Copper sulfate (CuSO4) can be used to treat marine fishes, but the concentration of active copper (Cu2+) must be closely monitored (test kits are available) and should be maintained at 0.18–0.2 mg/L for up to 3 wk. When using over-the-counter products in marine aquaria, label directions should be followed. Cu2+ concentrations should be tested at least once a day.

Copper is extremely toxic to invertebrates and plants, which must be removed before the water is treated. Finally, copper will adversely impact the nitrifying bacteria in biofilters, and a transient increase in ammonia and nitrite should be expected for weeks to months after treatment. Monitoring of ammonia and nitrite until measurable concentrations subside is recommended.

Organophosphates have been used in nonfood fish practice for decades to control monogenea, parasitic crustaceans, and leeches. Organophosphates can be used in freshwater systems, for ornamental fishes only, at concentrations of 0.25 mg/L as an indefinite bath. Most compounds are sold as 37.3% active ingredient liquid. Toxicity and efficacy are affected by pH, with more acidic pH resulting in increased toxicity. For this reason, an increased dosage may be necessary in marine systems (pH 8–8.3), with concentrations up to 1 mg/L used by some facilities. Some veterinarians add atropine (0.1 mg/kg, PO, IM, or IC) to the food of sensitive freshwater fishes and marine exhibit fish before treatment with organophosphates. Because of environmental concerns, organophosphates should not be used in outdoor ponds, unless specific provisions for such use exist in state law and are followed by the veterinarian. After treatment with organophosphates, most facilities hold water for up to 96 hr before allowing any discharge, and divers are usually restricted from entering exhibit tanks for at least 48 hr. Organophosphates may not be used in food fish in the USA.

Diflubenzuron is a chitin synthesis inhibitor effective against anchor worms (Lernaea), fish lice (Argulus), and other crustacean parasites in aquarium fish only. It is used as a prolonged bath at concentrations of 0.03 mg/L. It has a fairly long half-life (>1 wk), and treated water should be retained for 28 days and then run through a carbon filter before discharge.

Metronidazole is used to control intestinal protozoans and can be delivered in a medicated food or as a bath when fish are anorectic. Although very effective against Spironucleus spp, metronidazole does not seem to be effective against gastric infections with Cryptobia iubilans. A concentration of ~7 mg/L (~250 mg metronidazole dissolved in 10 gal. of water) can be administered daily for 5 days. A daily water change a few hours after treatment is recommended. Metronidazole can be administered in medicated feed at 50 mg/kg, PO, for 5 days. Metronidazole may not be used in food fish in the USA.

Fenbendazole has been used to control intestinal helminths in fish. A dosage of 25 mg/kg, delivered in food for 3–5 days, has been commonly recommended, but efficacy has not been evaluated in controlled trials. When fenbendazole has been used in a bath treatment, high mortality has occurred. Consequently, this use is not recommended. Levamisole administered in a bath treatment at a concentration of 2 mg/L is also used by some clinicians. These compounds may not be used in food fish in the USA.

Praziquantel is selective for flatworms, so consequently is used to control cestodes and external monogeneans. The most common use of praziquantel is as a prolonged bath in large marine aquaria for control of capsalid monogeneans. It is applied at a concentration of 5 mg/L and may remain active for several weeks. Treated water should be run through an activated carbon filter before discharge. Praziquantel can also be administered PO at a dosage of 35–125 mg/kg for up to 3 days or as a short-term bath treatment at a concentration of 10 mg/L for 3 hr. Efficacy has been demonstrated in yellowtail kingfish fed praziquantel at a dosage of 50 mg/kg/day, for 7 days. Use of praziquantel is not permitted in food fish in the USA.

Chloroquine has been used to control Amyloodinium spp in ornamental marine fish. It is applied as a prolonged bath at concentrations of 10 mg/L. Efficacy in recirculating systems seems to be very good; however, there are essentially no data on treatment intervals, effects on biofilters, or other basic husbandry effects. Weekly examination of fish, including biopsy of infected tissue, is recommended to assess treatment efficacy. Some aquarists have used chloroquine (10–15 mg/L for 7 days; follow-up with 10 mg/L may be required) coupled with decreased salinity (16–18 ppt) as a treatment for Cryptocaryon in marine fish. Results are mixed, but advantages include decreased labor, because intensive water testing is not required (as is the case for Cu2+). Use of chloroquine is not permitted in food fish in the USA. Treated water should be run through a carbon filter before discharge.


Tricaine methanesulfonate (MS-222) is the most commonly used anesthetic for fish. An FDA-approved product is available. MS-222 is used to sedate, anesthetize, or euthanize fish. It should always be buffered with baking soda (sodium bicarbonate) at a ratio of 2 parts baking soda to 1 part MS-222. See also Tricaine Methanesulfonate (MS-222, TMS).

Eugenol and clove oil (an over-the-counter product, usually 84% eugenol) have become popular with pet fish enthusiasts as anesthetics. These products are not FDA approved for use in fish in the USA, and use in food animals is prohibited. When eugenol at concentrations of 50, 100, and 200 mg/L was compared with MS-222 in red pacu, it resulted in effective immobilization of fish; however, there was concern about analgesia, prolonged recovery, and a narrow margin of safety, especially at higher concentrations. Use of both MS-222 and eugenol resulted in hypoxemia, hypercapnia, respiratory acidosis, and hyperglycemia in red pacu. Aqui-S® is a product approved for sedation of fish in other countries, and it is currently being investigated in the USA as an anesthetic for non-food fish. The active ingredient is isoeugenol, but evidence of carcinogenicity in laboratory animals has caused the use of this product in any food fish to be strictly prohibited. The US Fish and Wildlife Service has an active INAD program assessing the potential use of Aqui-S in non-food fish.


Veterinarians should follow the AVMA Guidelines for Euthanasia of Animals (see Euthanasia). When using overdoses of MS-222 for euthanasia, the pH should be carefully ascertained before use; the sodium bicarbonate buffering can be variable because of the type of water used to make up the euthanasia solution.

Surgical Considerations

Increasingly, surgery is an option for management of some medical problems in pet or exhibit fish, including neoplastic disease, failure to ovulate (ie, “egg bound” fish), and gas bladder repair for buoyancy problems. Fish skin does not have the subcutaneous tissue that provides flexibility to domesticated animal tissue; therefore, wounds are not usually treated by surgical closure but instead allowed to heal by second intention. The clinical evaluation before surgery is similar to that in other species, although there may be more emphasis on imaging and less on blood work. Radiology and ultrasound work very well in fish, and use of these techniques before an invasive surgical procedure is recommended.

Descriptions in the literature show how a fish may be positioned for a surgical procedure. For smaller animals, such as koi, a foam “bed” is easily constructed and can be covered with something as simple as clear plastic wrap so that the fish does not lose skin, scales, or mucus from direct contact with the foam. The “bed” can be positioned over an aquarium using a plastic tray with holes to allow water drainage. A pump can move water containing an anesthetic solution (usually MS-222) from the aquarium through a small tube or catheter and across the fish’s gills. Ideally, dissolved oxygen, ammonia, and pH should be monitored in the anesthetic solution. The fish can be covered with a clear plastic surgical drape (avian drapes work well), followed by fairly minimal preparation of the incision site. Plucking scales along the incision line and carefully cleaning the area with a sterile swab soaked in sterile saline may be all that is required. Very dilute disinfectants such as povidone-iodine or chlorhexidine can be used, but if the fish is clean this is probably not necessary (or recommended) because the normal mucus has significant antibacterial properties.

Absorbable sutures are generally not recommended in fish, because they may persist for significant periods of time (>1 yr in some cases). Monofilament material and a needle with a cutting edge generally work well. The simple interrupted suture pattern works well to close fish skin, but other techniques can be used successfully. Skin sutures should be removed when the incision site has healed, generally at 3–4 wk. Surgical staples have been used successfully in fish. Results using cyanoacrylate tissue adhesive have been mixed because of a significant inflammatory reaction seen in some species. If postoperative antibiotics are used, enrofloxacin delivered via IC injection at a dosage of 5 mg/kg is a good option but can only be given to non-food fish. Butorphanol given at 0.1–0.4 mg/kg, IM, and meloxicam at 0.15 mg/kg, IM, have been used for postoperative pain control in non-food fish. Increasing the salinity in freshwater systems to 1–3 ppt (g/L) is strongly recommended during the recovery and healing periods. Most freshwater fish should be able to tolerate a salinity of 3 ppt (3 g/L) on an almost indefinite basis.

Quarantine and Biosecurity

Quarantine is strongly recommended for pet fish and certainly for animals intended for display in public aquaria. A 30-day period is the minimum time for quarantine, but longer periods may be necessary. Quarantined animals should be handled only when contact with all other animals is finished for the day. Quarantined animals should have their own designated equipment (ie, nets, buckets, siphons, etc) and be kept separate from other animals. Disinfectant should be used on equipment and in footbaths at entry points to the quarantine area.

When receiving fish, a thorough history should be obtained regarding previous treatment or disease outbreaks. Fish should be examined early in the quarantine period; a visual examination may suffice, but for valuable specimens a full clinical examination, including recording the weight of the animal and completing gill, skin, and fin biopsies, is recommended. Moribund fish should be examined, and dead fish necropsied. Prophylactic use of antibiotics may be warranted in some shipments, especially recently imported marine specimens. Treatment with praziquantel for monogeneans is often prudent with marine fish as well. Goldfish commonly have significant monogenean infestations, and treatment with formalin or praziquantel may be appropriate. Koi should be quarantined to prevent introduction of koi herpesvirus (KHV), a serious and reportable disease, to established populations. Koi should be quarantined for a minimum of 30 days at a temperature of 75°F (24°C). Fish that become ill during quarantine should be tested for KHV (see Koi Herpesvirus).

Quarantine is not useful for some pathogens, but that does not mean it should not be done. Examples are mycobacteria, which at this time cannot be detected with nonlethal tests, and many viruses, microsporidia, and myxozoans. These diseases are usually detectable using necropsy techniques if quarantined animals that die are thoroughly examined. Quarantine is most useful for detection of external parasites and some internal parasites that can be detected by examination of feces.

Biosecurity for the Home Aquarium:

For a modest investment, a hobbyist can set up a quarantine tank using an inexpensive 10-gal. tank, sponge filter, small aeration pump, and heater. Once fishes have cleared quarantine, the tank and equipment should be disinfected and stored dry until needed again. During the quarantine period, separate nets and siphon hoses should be used exclusively for the quarantine tank. Before purchasing new fish, the hobbyist can run the previously disinfected sponge filter in an established tank that is free of disease to inoculate the sponge with nitrifying bacteria. This practice will help avoid the common water quality problems that occur in newly set up aquaria.

Common Water Quality Problems in Hobbyist Aquaria

New tank syndrome (see Aquatic Systems:Nitrogenous Compounds) usually occurs within the first 6 wk after a new fish hobbyist sets up a tank. Owners may report everything was fine for several weeks, and then the fishes suddenly become lethargic, anorectic, and frequently die. Although parasites may be present in newly purchased fishes, it is critical to perform a complete analysis on a water sample from the affected tank. Often total ammonia nitrogen (TAN) or nitrite, or both, are high enough to result in toxicity. Treatment should include decrease in feeding, water changes, the addition of chloride for nitrite toxicity, and evaluation of adequate biofiltration. It can take up to 8 wk for a biofilter in a tropical fish tank to become established. New tank syndrome can be avoided by several methods. One method, known as fishless cycling, is to completely set up the tank but with no fish addition; ammonia is added to achieve a concentration of at least 1–5 mg/L. This will initiate the “cycling” process, which should be monitored by regular testing for ammonia, nitrite, and nitrate. Once no ammonia or nitrite is present, fish can be added safely. Another method is to slowly add several fish to the tank over several months, although this can result in high ammonia or nitrite levels if not done carefully.

Old tank syndrome (see Aquatic Systems:Nitrogenous Compounds) can occur when water changes are small and infrequent. In some cases, no water is exchanged, and the practice of “topping off” is done by adding water to the tank to replace what has evaporated. Although this can occur in any tank, it is more frequent in tanks that are inhabited by large, carnivorous fishes. These fishes eat large, high-protein meals, frequently in the form of other fish, and consequently excrete large amounts of TAN. Consequently, the biofilter uses up bicarbonate to transform TAN to nitrite and nitrate, and the bicarbonate depletion decreases total alkalinity. This results in either a gradual or dramatic drop in pH. If the former, the fishes will adapt to the slow change often with no outward sign of distress. If the pH decline is rapid, the result could be death for the resident fishes. The water quality parameters of total hardness, nitrate, total alkalinity, and pH are often abnormal. Total hardness (TH) and nitrate are usually significantly higher than normal. TH of the tank water should be compared with TH of the water used for water changes. Total alkalinity and pH are usually significantly lower. Often TAN will be high because of inadequate bicarbonate for the nitrogen cycle. If the fish are still alive, the water quality parameters should be returned to normal by performing daily small water changes to avoid pH shock. From that point on, the tank should receive regular, large water changes to prevent a recurrence of the problem.

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