Saltwater Fish Medicine
American Association of Zoo Veterinarians Conference 2013
Meredith M. Clancy, DVM
Zoological Health Program, Wildlife Conservation Society, Bronx, NY, USA


Fish are common pets, with an increasing trend toward sophisticated marine aquaria. Estimates indicate that over 80% of fish owners also have other pets, so the veterinary-client relationship may already exist for the exotic animal practitioner looking to expand their business into fish.

Fish are extremely diverse. For the purposes of this presentation, they are primarily divided based on their environment's salinity. Freshwater fishes live in water that has < 0.5 parts per thousand (ppt = g/L) salt. Saltwater or marine fishes live in water between 30 and 40 ppt salt.5 There are fish that live in brackish (0.5–30 ppt salt) water, and fish that can move between brackish and fresh or salt water, but few fish have the capability to live in both salt and fresh water systems.4 Issues that arise with improper salinity are one example of the importance of water quality to a fish's health. Saltwater fish are also generally less forgiving of environmental disturbances like temperature fluctuations than their freshwater counterparts.

Many of the topics discussed in this presentation are applicable for freshwater ornamental fish, but the focus of this presentation, including water quality parameters and diseases, will be on marine or saltwater fishes. Clinical techniques and some important concepts, however, cross the salinity barrier and are applicable to all fish an exotic animal practitioner may see.

Saltwater fish may be kept in large numbers in captivity, but their appearance at the veterinary hospital is relatively rare. This presentation will approach saltwater fishes and their medicine by two ways: environmental concerns, including water quality that can affect the entire system, and individual problems affecting a single fish.

The most commonly seen marine fish will be tropical fish kept in groups in larger systems, such as the "rapid eaters" like the angelfish, damselfish, triggerfish, and groupers. Smaller systems may feature "slow eaters" like the clown fish, parrotfish, puffer fish, surgeonfish, wrasses, and trunkfish. Seahorses and other more unusual fish are kept far less commonly. Marine coldwater fishes are generally too large for private aquaria, and many are commercial food fish, such as cod, hake, pollock, and tuna, or the flatfish, like flounder and halibut. Elasmobranches are the final grouping of marine fishes, and include sharks and rays, which are far rarer as private practice veterinary patients.4 The marine invertebrates that share a system may present to the veterinarian, with anemones, sea urchins, starfish, shrimp, crabs, and corals as possible patients. The tank system makes fish medicine a bit akin to herd health in this regard.5

Understanding Saltwater Fish Anatomy and Physiology

The unique anatomy of the plethora of saltwater fishes defies complete description in this presentation. The most important differences between the common vertebrate anatomy and the anatomy of the fish are the respiratory system, the integument, and the special senses. Fish respire through gills, which absorb oxygen and excrete ammonia and carbon dioxide. Gills are found behind specialized flaps known as opercula that generate the flow of water over the gills to produce the countercurrent exchange necessary for respiration and excretion. The fish is covered in scales located in the dermis. A mucous coat covers the scales. The handling veterinarian should take care not to strip the mucus or damage the scales as this can lead to opportunistic infections. Moistened, powderless gloves should be worn when handling fish for this purpose. Fish have a mechanosensory structure, the lateral line that senses movement in the water through alterations in sound waves and water pressure.4

The physiology of an animal living constantly in a hypertonic milieu is necessarily complex. The gills, kidneys, urinary bladder and gastrointestinal tract all function to maintain fluid and electrolyte homeostasis. Fish convert nitrogenous wastes to ammonia, excreted by the gills and kidneys. Fish have two kidneys, an anterior (head) kidney located just behind the gill arches and associated with hematopoiesis, and a posterior (tail) kidney more akin to other vertebrate kidneys.6 To fight against a constant osmotic loss of water to their environment, the marine fish is constantly drinking salt water, leading to very high loads of sodium and chloride for excretion. While the kidneys are important sites of electrolyte regulation, the gills are possibly even more so, with a large amount of energy used to maintain electrolyte balances.6 Other anatomic considerations include the swim bladder, evolutionarily derived from the gastrointestinal tract, and in certain fish, like pufferfish, the communication still exists. The swim bladder is thought to function in respiration and hydrostatic equilibrium, with possible involvement in sensation of pressure or sound production. Swim bladder pathology is one cause of irregular buoyancy.6 Fish have nucleated erythrocytes. Leukocytes are either agranular, similar to lymphocytes and monocytes of other lower vertebrates, or granular, like heterophils or eosinophils.4 The gonads, especially in the female, can enlarge when active to fill most of the coelom.6

Water Quality

Perhaps the single most important factor for a fish's health is the quality of the water in which it swims. Water parameters that must be tightly regulated for saltwater fishes include temperature, lighting, oxygenation, pH, hardness, alkalinity, nitrogenous wastes and chlorine (Table 1). These are regulated through appropriate natural cycles and filtration of the tank. Filtration comes in three main mechanisms:

  Mechanical filtration removes organic debris through a porous material. Pore size, filtration rate and filter size can vary the amount of filtration mechanical filters perform. This method is widely employed in home aquaria, but requires maintenance and has its limitations. It is best used in combination with one of the other methods described below.

  Biologic filtration is performed by nitrifying bacteria. Nitrifying bacteria denature the ammonia excreted by fish into nitrite, which is further reduced to nitrate, as both ammonia and nitrite can be toxic when allowed to build up in a closed system. The nitrifying bacteria colonize objects that serve as the biologic filter, like an under-gravel filter or bead filter. Temperature, oxygenation, and antibiotics in the system can affect the bacteria.

  Chemical filtration comes in many forms, but generally consists of activated carbon that can nonspecifically bind numerous toxic compounds. UV sterilization and ozonation are two other forms of chemical filtration.4

Water quality testing is something the exotic animal practitioner seeing fish should be able to perform. Most pet owners who are willing to bring their fish to the vet will have water quality testing ability, but ask the owner to bring a sample of the tank's water with them to test in the clinic. Major parameters to examine are as follows:

  Ammonia, nitrite, and nitrate - as discussed above, ammonia and nitrite are toxic in a closed system, and elevation in these levels points to inadequate filtration, buildup of wastes due to lack of physical cleaning of the system (i.e., gravel-washing), or stocking densities that are too high.

  pH - measurement of dissolved H+ in the water. Like temperature, pH is ecosystem and species specific. Keep in mind that there is great variation between even one point difference in pH due to the logarithmic measurement of pH.

  Alkalinity - measurement of buffers in the system, in the form of bicarbonate (HCO3) and carbonate (CO32-) and other buffers.

  Hardness - measurement of the divalent cations (Ca2+, Mg2+) in the water.

  Chlorine - often added to municipal water, and toxic to fish if not removed prior to adding to the tank. Removed via dechlorinating drops or by "gassing out" the water for 24 hours prior to use.

  Oxygenation - measured as dissolved oxygen (DO); generally maintained in home aquaria via aerators or bubblers. Too high of oxygenation can be as problematic as too low.4


In addition to water quality, history and visual exams are important first steps in examining an individual or system patient. Ruling out water quality problems first, find out if there have been recent additions to the system or problems with filtration, power, temperature, or any other changes that could have thrown the system out of order.5 Many pet owners do not properly quarantine their new fish, which can be a source of disease as in all species. If able, try to observe the fish - is it gasping for air at the top of the water column? Dark in color? Gilling quickly? Does it appear to have altered buoyancy? Assessing fish body condition takes practice, but can be an excellent way to gauge chronicity. Fish, like many veterinary patients, try to hide any health concerns until the disease process is very far along, making acute observation essential. Only when you have a full picture of the fish in its environment should you start to collect diagnostics.

This author's core diagnostics for a fish generally include a gill clip and skin scrape +/- fin biopsy. Blood samples from larger fish can be contributory, and for any buoyancy issue, radiographs are recommended. As you are literally taking the fish out of water to perform many of these tasks, it is essential to have your supplies ready and diagnostics planned prior to handling the animal. Sedation or anesthesia may be used, but often, core diagnostics can be collected without chemical restraint, with radiographs the exception.5 Tricaine (MS-222) is the recommended and most widely used fish anesthetic agent. It is added to the water at a dose of 50–100 ppm to achieve reasonable sedation for most procedures with levels increasing for 100–200 ppm for induction to surgical anesthesia.5,6

Gill clip: Gently restraining the fish with the operculum elevated using the thumb or index finger of the restraining hand, the rows of the gills are separated and a small sample is collected from one row of distal primary lamellae and immediately placed on a slide with a drop of the fish's ambient water. A cover slip is placed on top, and the slide examined on low magnification (10x for scanning, 40x for parasite ID).

Skin scrape: Using a slide cover slip, scrape the edge at a 45-degree angle in the direction of the scale growth. Excellent spots for catching parasites include just caudal to the pectoral fins and behind the opercula. Scrape any areas of discoloration found on the skin. Multiple samples, including a dry mount for possible Gram or acid-fast staining should be collected of wound sites. Scales and mucus should be present on the coverslip and transferred to the slide for wet mount and examination under low power.

Fin biopsy: Using similar small (iris) scissors as used for the gill clip, cut a small wedge out of the tail or a large fin. Include any areas with spots, irregularities or wounds. Place the resected area on a wet mount slide and examine.5

Blood sampling: The most reliable location for blood collection is the caudal tail vein. The needle is positioned at a 90-degree angle to the fish's vertebral column with the fish in lateral recumbency. The needle is inserted until it contacts the vertebral column and then is "walked down" off the vertebral column until just caudal, where the vein should be found. Maximum collection amount is approximately 1% of body weight.5

Pathology: Necropsy is the ultimate diagnostic. All the above samples can and should be collected post-mortem, especially in an animal that has died out of a tank system with other animals. Gross necropsies can be extremely rewarding in finding major diagnoses on saltwater fishes, and if you are planning on seeing these animals, you should become familiar with standard fish necropsy protocol.5-6

Major Diseases of Saltwater Fishes

Diseases that infect saltwater fishes can be categorized by the causative agent. Many causes of illness are caused by noninfectious or environmental agents, as indicated in the water quality discussion above. A major consideration for marine fishes is their origin. Most marine fishes are sourced from the wild. In addition to its effects on the wild environment, this practice tends to introduce a variety of marine parasites and other disease processes. Even the clownfish purchased at a pet store may have recently been swimming in the ocean, again underscoring the importance of quarantine for home reef tanks.

Noninfectious diseases of note: When the total pressure of dissolved gases in a water system is higher than the ambient atmospheric pressure, this is called gas supersaturation. This excess gas can leave solution to equilibrate with the atmosphere, resulting in bubbles in the blood vessels of a fish, called gas bubble disease (GBD). Nitrogen gas is the most common cause of gas emboli. The emboli commonly occur in the gills and eye.4-6 The severity of signs is the greatest prognostic indicator, along with duration and response to treatment. Affected fish should be moved to a separate system if possible; the gas emboli can be removed via syringe in some cases, and many other treatment modalities, each with varying levels of success exist to treat GBD.4

Lateral line disease or depigmentation occurs in fresh and saltwater fishes and comes with a bevy of synonyms and proposed causes. It presents as coalescing foci of depigmentation along the lateral line of the fish especially near the head. In marine fish it is most common in tangs and angelfish. It is chronic and very difficult to reverse with treatment.5

Infectious diseases: Marine fishes have numerous important bacterial pathogens, including those in the genera Edwardsiella, Vibrio, and Streptococcus. Edwardsiellosis, or red pest disease, is caused by E. tarda and can result in mass die-offs due to acute septicemia. Diagnosis is generally made via culture from post-mortem sampling of the kidney. In systems where an animal has died with a culture positive for E. tarda, treatment may include systemic antibiotic treatment, reduced stocking density, and increased water filtration.2 When considering antibiotic therapy, bear in mind that regulation of aquaculture by the FDA places extra regulatory burden on antibiotic use in fish.1

Parasitic diseases: Parasites are important sources of morbidity in captive marine fishes. Parasites are one of the main reasons behind quarantine recommendations, as once a marine parasite is introduced to a tank, clearance is nearly impossible. Protozoa represent the most common parasites in marine tropical fishes.4 Signs of protozoal parasitism can be as subtle as increased skin mucus, flashing, and spots on the skin or fins, or as severe as sudden death. Understanding the life style of the pathogen is essential for adequate treatment. For nonencysted protozoans, one or two doses of treatment is effective. The encysting protozoans like Cryptocaryon irritans produce a reproductive cyst off the fish host; both the encysted protozoan and its reproductive cyst are resistant to treatment, so longer-term therapy must be used to target the free-swimming infective stage.5 Cryptocaryon is the white spot disease in marine fish, and a common malady of captive aquaria fish. It has a similar pathology to the freshwater protozoa, Ichthyophthirius multifiliis or "Ich" and is sometimes called marine Ich.5

Numerous treatment modalities exist using products sold over the counter to hobbyists, including additives containing copper, nitroimidazole, and malachite green. Formalin and copper can be very efficacious in treating marine parasites, but formalin is a hazardous material and copper has a narrow safety margin. Both should not be used in tanks with live corals or scaleless fishes. Entire presentations could be given about the variations of treatment for parasites depending on species and system, but the same protocol is often used for all parasitism. The astute clinical practitioner should find a product with broad effectiveness and become comfortable with that product as their first line treatment. An important principle for any treatment choice is to always know what the active ingredient is, and know how to manage filtration and water changes. Carbon filters should generally be taken offline when performing an immersion treatment. Water changes of 25%–50% allow for elimination of the active ingredient at the cessation of treatment.4-6

Most importantly, if you are faced with an unknown parasite or complicated case, it is best to ask for help early in the course of treatment. Using programs like Fishbase ( and your colleagues as resources, you can help to shape the appropriate therapy for each individual and system.

Table 1. Water quality parameters for saltwater fish


Measurement (units)




0.00–0.03 (ppm)


0.00–0.10 (ppm)


< 25 (ppm)


120–200 (ppm CaCO3)


350–475 (ppm Ca2+)


1.  Aquaculture: approved drugs. 2012. Food and Drug Administration. Accessed 15 August 2013.

2.  Hadfield, C. A. 2011. Edwardsiellosis. In: Napier, J. E., and K. C. Gamble (eds). Infectious Diseases of Concern to Captive and Free-Ranging Animals in North America. 1st ed. Infectious Disease Committee, American Association of Zoo Veterinarians, Yulee, Florida. Accessed 15 August 2013.

3.  Hadfield, C. A., and L. A. Clayton. 2011. Fish quarantine: current practices in public zoos and aquaria. J. Zoo Wildl. Med. 42: 641–650.

4.  Miller, S. M., and M. A. Mitchell. 2009. Ornamental Fish. In: Mitchell, M. A., and T. N. Tully, Jr. (eds.). Manual of Exotic Pet Practice. Saunders, St. Louis, Missouri. Pp. 39–72.

5.  Noga, E. J. 2010. Fish Disease: Diagnosis and Treatment. 2nd ed. Wiley-Blackwell, Ames, Iowa.

6.  Stoskopf, M. K. 1993. Fish Medicine. W.B. Saunders Co., Philadelphia, Pennsylvania.


Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Meredith M. Clancy, DVM
Zoological Health Program
Wildlife Conservation Society
Bronx, NY, USA

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