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Authors: Roxanna Smolowitz, D.V.M., Joerg Mayer, D.V.M.
Color Key
Important key words or phrases.
Important concepts or main ideas.

OCW Zoological Medicine 2008
Fish Medicine (2009)
R. Smolowitz, DVM / J. Mayer, DVM
Cummings School of Veterinary Medicine at Tufts University

1. Session Objectives and Goals

This session will highlight significant anatomical and physiological differences between fish patients and the terrestrial and avian patients commonly seen by the exotic veterinarian. It will provide information concerning the environment potentiation of disease as well as diseases commonly identified in fish.

Fish are becoming more and more popular as pets as well as exhibit animals in aquariums and zoos and important research animals. There is need for veterinarians who either specialize in aquatic medicine or make it a part of their multispecies abilities. This lecture is aimed at providing a baseline of knowledge to students interested in aquatic fish medicine.

1.1. Learning Objectives

Color coded topics indicate learning objectives that the student should become familiar with. At a minimum, you will be expected to be familiar with the following:

  • Gain an appreciation for the key organ systems in fish responsible for excretion, respiration and the control of osmolality

  • Know the main physiological differences between saltwater and freshwater fish

  • Gain a basic understanding of the importance of maintaining good water quality for aquarium fish and how this is achieved

  • Be able to develop a basic clinical approach for a diseased fish presented for examination

  • Gain an awareness of the environmental concerns affecting aquaculture and wild fish stocks

1.2. Suggested review for this section

Please review the 1st Year Comparative Anatomy notes on fish prior to attending lecture. Another great site for anatomy review can be found at Comparative Organology

References and Resources

Conservation Medicine Challenges

Supplemental Readings


2. Fish Anatomy

There are approximately 20,000 different fish species currently known to science. The anatomy and physiology of these many species highlights the adaption to the aquatic environment each inhabits.

Fish liver and gonads are the easiest organs to identify upon dissection. All fish lack organs seen in mammals and bird such as lymph nodes, lungs, and bone marrow. In many fish other cells are present which would form distinct organs in mammals, but in fish are diffusely found associated with other organs or tissues. These include cells of pancreatic, islet, thyroid and adrenal origin. Other organs may be present but are anatomically different in form to their mammalian equivalent, e.g., kidneys, gonads, skin, heart. Additionally, other anatomical features are present in fish but not in mammals or birds. These include fins, the lateral line organ, swim bladder, and the gills.

Leafy seadragon

2.1. Fins

The fins are one of the most obvious anatomical features on the fish and they serve a multitude of tasks in addition to locomotion. The dorsal fin is usually used as a stabilizer during swimming but it can also be used during courtship rituals or for defense. Many species have sharp projections protruding from the dorsal fins and it is important to protect these from damage during handling as well as to take care to prevent them from injuring the handler. The pectoral fins are used to counteract forward motion during swimming and act as a 'brake'. The pelvic fin, as well as the anal fins, act as a stabilizers. In males the anal fin can differentiate anatomically into a male sex organ, the gonopodium. This organ is easily observed in many live-bearers e.g., guppies, mollies, and swordfish.

2.2. Integument

Just as in terrestrial animals, the skin of fish provides a barrier to the world around it. In bony fish scales, classified as cycloid, ganoid and ctenoid, are produced in the dermal layer and are composed of mineralized structures that are of mesodermal origin. Epidermal cells completely cover these bony plates and the epidermis varies greatly in thickness depending on the fish species. In contrast, the placoid scales of members of the Chondrichthyes originate in the dermis but have a structure eminence of a tooth. There is a pulp like space at the base of the scale and the scale’s spine projects through the surface of the epidermis.

The slime, or mucus layer, is a vital protective coating partially produced by mucus containing goblet cells in the epidermis. These cells can even produce cocoons for wintertime aestivation! The slime layer has both antibiotic and antifungal activity and acts as the initial defensive shield against many pathogens. When handling fish, it is very important to protect their slime layer. The skin of some fish also contains other specific cells, such as alarm cells (pheromoes), ionocytes and taste buds.

2.3. Circulatory System

The hematopoetic tissue of fish occurs primarily in the interstices of the anterior kidney, but cells are also produced in the spleen and the liver. Blood cells in fish are similar to those of the reptilian or avian patient. Normal blood cell parameters are lacking for many fish species but fortunately more and more data is being published. It is not possible to extrapolate fish normals from terrestrial models as a leukocyte count of only 10% for example might be normal in some fish. Lymph vessels exist but there are no separate lymph nodes or defined lymph circulation system. The heart is positioned just caudo-ventral to the gills. It consists of 4 chambers: the sinous venosus, one atrium, one ventricle and the bulbus arteriosus. These chambers are arranged linearly and blood circulates in a single circulatory pathway through the heart. The heart can be accessed for phlebotomy, however the preferred site is the caudal vein.

2.4. Urinary System

The gills and the kidney make up the major excretory organs of the fish. The end product of nitrogen metabolism in fish is ammonia. The gills can excrete up to 75% of the ammonia load. Some fish also have a urinary bladder. A more detailed discussion of the gills and kidney is covered in the physiology part of this lecture.

2.5. Gastrointestinal System

The barbels which many fish have, including koi, serve a double function. They are a tactile as well as a gustatory organ. Fish lack a muscular tongue; however, teeth and taste buds may be present in the oral cavity. While some fish have a fully functional stomach, certain other stomachs lack histological differentiation and serve purely as a storage organ. In general herbivorous fish have longer GI tracts than carnivorous fish. Since fish are poikilothermic digestion is very dependant on the environmental (water) temperature.

Cory aeneus
Cory aeneus

2.6. Respiratory System

The respiratory system of fish can be divided into three cavities. The buccal cavity and two opercular cavities. The buccal cavity is separated from the opercular cavity by two valves named the mandibular and maxillary valves that open and close with the mouth. Most fish have four gill arches on each side within the opercular cavity. Cartilagenous rods stables the arch centrally and gill rakers extend anterio-medially into the pharyngeal cavity. Extending laterally from each arch are two lamellae (hemibranchs). Each primary lamella is further subdivided into leaflets (secondary lamellae). An additional structure found in most boney fish is the pseudobranch. This flattened organ is located anterior-dorsally on the operculum and is somewhat similar to a secondary lamallae.

Sharks, skates and rays have a single orobranchial cavity that divides into 5 separate parabranchial cavities on each side of the head or on the ventrum of the body. Skates and Rays use a spiracles (one behind each eye) to draw in water that then flows over the gills and exits through the gill slits on the underside of the animal.

3. Fish Physiology

In this lecture we will focus on only two physiological processes, respiration and excretion. Understanding these two processes is extremely important to the general understanding of fish physiology. In the fish, respiration and excretion are very tightly linked and the gills are key players in both processes.

3.1. Respiration

As in the terrestrial patient, oxygen exchange is the primary goal of respiration in fish. The physiological process of extracting oxygen from water is much more difficult than extracting oxygen from air. Two major factors are responsible for this. Water is approximately 800 times denser than air and contains only about 3% oxygen. In contrast air usually contains approximately 20 % oxygen. The process of respiration is extremely energy consumptive and the system can only work well if the fish is good physical condition and the environment contains adequate dissolved oxygen. In addition, the surface area of the gills is only about 6-10 times greater than the surface area of the entire body. This is a relatively small difference in comparison to the relative size of the lung as an exchange organ. In contrast, the lung surface is usually 100 times the body surface in mammals. Gas exchange happens in the secondary lamellae of the gills and is extremely efficient. This efficiency is achieved by the countercurrent flow of water and blood. The oxygen-poor venous blood moves opposite to the flow of the relatively oxygen-rich water. In this mechanism, water must flow constantly over the gills in order to keep the respiration effective. About 80% of the environmental oxygen is removed during respiration. In humans only 25% of the oxygen is usually removed from the air during respiration. In fish, anesthesia is achieved using these principles. The anesthetic agent is dissolved in water and anesthesia is maintained by keeping the medicated water flowing across the gills, even if the rest of the fish's body is out of water. The extreme effectiveness of the gills as gas exchange organs also makes them extremely vulnerable to toxic insult.

3.2. Excretion

As mentioned above, the gills are also one of the main excretory organs. The gills excrete the majority of the ammonia while the rest of the waste products are excreted via the kidneys. The excretion of metabolic waste products is similar in all fish; however, the kidney and the gills play significantly different roles in fresh water fish in comparison to their roles in salt water fish.

The freshwater fish is hypertonic in comparison to the environment. As a direct consequence, water is constantly entering the fish's body via the gills and diluting the blood. Therefore in fresh water fish, the major role of the kidneys is to eliminate the excess water from the circulatory system. In addition, electrolytes must be conserved during the elimination process. Freshwater fish therefore have relatively large glomeruli within the kidney. The situation is exactly the opposite in marine fish. The salt water fish is hypotonic in comparison to the marine environment. Marine fish constantly need to drink water, as water is constantly lost from the gills into the environment. The major work of the kidney is therefore to conserve water and to eliminate electrolytes. For that reason certain marine fish have aglomerulic kidneys.

The plasma of sharks, skates and rays is slightly hyperosmotic compared to their water environment. This allows for a slight influx of water. Urea and trimethylamine oxides are produced by the liver and excreted into the plasma in order to raise the osmotic pressure of the plasma. A rectal gland which empties into the rectal lumen eliminates excess sodium and chloride from the blood.

4. Basic and Advanced Fish Medicine

Basic techniques start from simple observation of behavior and other hands-off procedures, to collection of samples such as fin and gill clips and phlebotomy. Advanced techniques include imaging techniques such as radiographs, ultrasound exams, and invasive sampling techniques such as biopsies and surgical procedures.

4.1. "Hands-Off" Procedures

Just as body temperature, heart and respiratory rates are some of the most important data collected during the clinical exam in mammals, establishing water chemistry values and making simple observations of the fish in the water are of paramount importance when assessing the fish patient. Observations from the tank side will frequently reveal abnormalities. Increased respirations, spiraling, difficulty keeping position in the water column, etc., should be noted. Some frequently observed abnormal morphological features in the fish patient include exophthalmus (pop-eye), fin necrosis (fin rot), bloody fins, dermal ulcers, excessive slime layer, and cataracts. Unfortunately, while all of these signs are significant abnormal findings, they are also very non-specific. Each of these clinical signs can be caused by a multitude of different infectious and non-infectious causes. In order to correctly diagnose a possible cause, 'hands-on' procedures are required.

4.2. “Hands-on” Procedures

4.2.1. Skin smear

If an excessive amount of slime is noticed on the fish, and the water chemistry results are well within normal values, it is advisable to examine some of the slime under the microscope for the presence of ectoparasites. This skin smear examination can also be part of a routine health exam in order to pick up sub-clinical levels of parasites. A good sample of the slime layer can be collected by swiping a cover slip over the scales in a cranial-caudal direction with a 30-45 degree angle in the direction of the movement. The coverslip should be placed on the slide and read immediately since many ectoparasites will eventually die or stop moving, making their detection more difficult.

Skin scraping
Skin scraping

4.2.2. Fin clip

If mild or severe lesions of the fins are observed, a fin clip may reveal the causative problem. A fin clip can be done in the awake patient and will usually not need hemostasis. With a very small pair of scissors, a small sample of the tip of a fin (dorsal, anal, or caudal fin) is clipped and evaluated under the microscope. A cover slip can be used. A drop of saline should be applied to the slide, if needed, to better visualize the tissue.

4.2.3. Gill clip

In a case of respiratory distress, when lesions involving the gills are observed, or even as part of a routine health check, a gill clip can be performed. In order to avoid traumatic injuries to the patient, it is advisable to perform a gill clip under general anesthesia. A small sample of the gills can easily be collected from one of the gill arches, and again, hemostasis is usually not required if the procedure is done in a controlled fashion. A gill arch can be lifted up from the rest of the collapsed gills and the sample taken with one clip. The sample is then placed on a slide, immersed in 1-2 drops of water from the tank or saline, and then covered by a cover slip and carefully crushed. The microstructure of the gill can be observed for chronic or acute changes (e.g., excess slime layer, parasitic damage, necrosis etc.).

Gill biopsy
Gill biopsy

4.2.4. Phlebotomy

While we still lack significant data regarding the blood values and hemodynamics in fish, more and more data is being published in this specialty. Even without reference values blood collection can be useful in the clinical work up. The easiest way to obtain blood from a fish is from the lateral tail vein. The fish is held in lateral recumbency out of the water and the needle is inserted at the midline of the caudal lateral aspect of the body. The needle is advanced up to the spinal column and under negative pressure slightly withdrawn. The technique is very similar to the lateral approach to the lizard tail. Blood should be collected in a lithium heparin tube as the blood of some species may react with the Ca-EDTA.

Fish blood smear
Fish blood smear


4.3. Imaging Techniques

4.3.1. Radiography

Just as with the avian and mammalian patient, the radiograph can be an extremely useful tool in aquatic medicine. Full body radiographs can be done with the patient submersed in water (in a bucket) or with the fish out of water. The author prefers the fish to be out of water, as this techniques yields superior image quality (i.e., higher detail). If the fish is taken out of water, care must be taken not to damage the delicate skin or to disrupt the slim layer. A moist paper towel or a moist plastic bag can be used as a protective layer between the fish and the radiographic plate. The lateral view can be achieved with the fish in lateral recumbency on the radiographic plate. If the fish has to stay in the water, a narrow container (e.g., fish corral) and a horizontal beam image can be taken with the plate behind the container. The DV image can be achieved with the fish swimming in the bowl and the plate placed under the bowl. If this technique is used, restrict the movement of the fish as much as possible especially if high detail settings are used, since they need a longer exposure time. For better images, the fish can be 'propped' up between two foam blocks. Larger koi can even be placed on the plate and the pectoral fins used as lateral support (fish needs to be very sick for the latter techniques (i.e., not moving at all).

Basic knowledge of fish radiographic anatomy is extremely helpful when interpreting the images. Due to the large number of different fish species seen in the clinic, a large variety of anatomical features can be observed (e.g., bi-lobed swim bladders in the carp family vs. one-chambered swim bladders in most species, and some bottom dwellers with no swim bladders). Otherwise, routine principles of radiology (e.g., check for symmetry, read in organized direction) should be applied when reading the radiograph. If not certain about specific details, digital images of the image can easily be obtained and emailed to an expert for second opinion..

Fish radiography
Fish radiography

4.3.2. Ultrasound

While an ultrasound examination of a fish appears to be strange initially, it can often provide the clinician with very valuable information about the aquatic patient. Water is the best medium for the ultrasound to travel through, and without an air interphase, the images are usually extremely clear and free of artifacts. One of the most important and useful features of an ultrasound exam in the fish is the ability to examine the coelomic cavity for the presence of free fluids. If fluid is found, ultrasound guided aspiration of the fluid can be performed. This will often provide immediate relief for the patient and the fluid can then be cultured and submitted for analysis. The kidneys can be examined and checked for polycystic kidney disease, coelomic swellings can be differentiated from a mass, egg binding, dropsy, organomegaly or other complications.

Fish ultrasound
Fish ultrasound

4.3.3. Other imaging

Other imaging such as computer tomography, magnetic resonance imaging and radioactive scans have also been used in the past as diagnostic tools in the aquatic patient.

4.4. Other procedures

Many advanced diagnostic, and all surgical procedures, require anesthesia in fish. For anesthetic drugs and protocols one should refer to the references. MS222 (Finquel) is the most commonly used anesthetic drug in fish.

Nearly every surgical approach used in mammalian medicine has been used in aquatic medicine. Scalpel blades can be used to remove dermal tumors and to gain access to the coelomic cavity. Laser surgery and cryosurgery has been used successfully in fish.

5. Common Problems of Fish

5.1. Water Quality

Without a doubt, maintaining good water quality is the key to success in fish health. Basic water quality parameters that need to be monitored include: temperature, dissolved oxygen, ammonia, nitrite, nitrate, pH, and water hardness. Other parameters such as levels of iron and other metals, and salinity are also important. While interactions and changes within the system are complex and very dynamic, the fundamentals of the water chemistry need to be understood in order to help resolve clinical cases and to prevent future diseases. The ammonia cycle is the most important environmental component in the aquatic system and will be discussed in detail.

5.1.1. The Ammonia Cycle

A common mistake of the novice aquarist is to set up their aquarium system and stock it before allowing the ammonia cycle to go through all its phases. The result is usually large fish losses due to build up of toxic waste products before the system has developed the capability of handling them. This is often referred to as 'new tank syndrome'. The normal initiation of the ammonia cycle follows a slow process over several weeks during which populations of appropriate bacteria to develop to which convert waste products into less toxic compounds. If the system is appropriately set up with plants and one or two fish, ammonia levels will normally rise due to the breakdown of fish excrement, dead plant material and food leftovers. If only a few animals are present this will not reach high enough levels to injure them. Slowly, after about 6-8 weeks a bacterial population (Nitrosomas sp.) will build up and transform this ammonia into nitrites which are slightly less toxic than the ammonia. Eventually a second population of bacteria (Nitrobacter sp.) will develop which will change the nitrites into nitrates, which are the least toxic byproduct and are not very harmful to the fish.

5.2. Disease Etiology

Due to their multi-factorial character, diagnosing fish diseases can be very confusing and frustrating. Some diseases manifest in fish with multiple different clinical signs (e.g., Mycobacterium sp.) while other diseases of different origins all manifest with the same clinical signs (e.g., cataracts). Therefore, identification of both the causative agents as well as any predisposing conditions are both important in disease control and long-term prevention. When developing an understanding the dynamics of fish disease it is extremely important to consider the physical properties of the water since this is the primary environment of fish, and fish are even more closely influenced by their immediate environment than terrestrial animals are by the air in which they live. Water transports many pathogens more efficiently than air, and water will also keep pathogens alive better than air (drying is one form of sterilization). Therefore consider that fish are constantly immersed in a bacterial soup.

5.2.1. Stress

Stress is the sum of the biological reactions to any adverse stimulus, physical, internal or external, that disturbs the homeostasis of an organism. Should these stress reactions be inappropriate, they may lead to disease states.

As one can imagine, inappropriate husbandry conditions translate into stress on the fish. As examples of stressors we should always think of improper housing (lighting, humidity, temperature, water quality, improper hiding places), improper social structure, improper food, inadequate cleaning, etc. In order to avoid stress due to these factors it is of paramount importance to know your animals' natural history. Typical reactions to stress in fish are seen as decreased reproduction, decreased feeding, decreased immune function leading to disease and eventually death.


As an example, social structure can be an important consideration. Contrary to common belief, fish can be very aggressive and are usually very territorial. Tank mate aggression is not uncommon. If fish do not get along in an aquarium, the more aggressive one will often try to chase the others out of its territory. This constant chasing and consequent stress will eventually kill the subordinate fish.

5.3. Common Problems of Captive Fish

5.3.1. Husbandry

  1. Providing an inappropriate environment.

  2. Failure to establish the nitrogen cycle in the tank “new tank syndrome”.

  3. Overstocking, leading to traumatic injury, territorialism, and cannibalism and oxygen shortage.

  4. Overfeeding leading to ammonia overload in the tank.

  5. Failure to properly quarantine new additions.

  6. Failure to remove dead animals or decaying food and plants.

  7. Use of bathroom silicon (impregnated with insecticides and fungicides)

  8. Use of toxins around aquaria (floor stripping, pest control, cigarette smoke, bleach).

  9. Failure to separate certain species (solitary vs. communal fish)

  10. Failure to check the water chemistry on a regular basis (dynamic system).

  11. Inadequate nutrition (underfeeding or inappropriate diet).

  12. Misinterpreting reproductive or other normal behavior as aberrant.

  13. Use of metal piping that can corrode and leach toxic salts.

  14. Improperly secured electrical equipment, frayed wires (pond pumps, lights, etc.) leading to electrocution.

5.3.2. Common Infectious Diseases

For a good diagnostic work up, it is necessary to work with a living fish or a freshly killed specimen. Parasites

It is very important to examine live fish for parasite diagnostics! External parasites will leave the host or die when the fish dies and therefore will be unavailable for diagnostic exams.


Commonly encountered external parasites:

  •  Ichthyophthirius multifiliis (Ich)

  •  Trichodina spp

  •  Lernaea spp. (Anchor worm)

  •  Argulus spp. (Fish lice)

Diagnostic techniques for common external parasites

  • Skin scrape

  • Gill clip


Animals infected with ectoparasites are often irritated by them and may therefore show

abnormal behavior patterns (e.g., flashing, scraping) and an excessive amount of mucus over their body.




  •  Salt, non-iodized (or fresh water, if marine fish)

  •  Copper

  • Ivermectin

  • Praziquantel

  • Formalin Bacterial Diseases

Remember that fish live in a bacterial soup and are constantly challenged by the presence of these organisms. Most infections are opportunistic. Bacteria can cause dermatitis, dermal ulcerations, branchitis, and bacteremia. Secondary bacterial infections often follow other primary lesions.

Fin rot is a non-specific bacterial disease involving opportunistic bacteria. It causes dermatitis of the fins and is often seen in newly acquired fish. It is usually self-limiting, resolving as they settle in or as water quality is improved. Fin erosion demonstrates just how sensitive fish can be to stress, and its presence is often a sign that all is not well.

Common opportunistic bacteria:

  • Flexibacteria spp.

  • Aeromonas hydrophila

  • Vibrio spp

  •  Edwardsiella spp.


Diagnostic methods:

  • Culture of dermal ulcers, blood, liver and kidney

  • Squash prep of the gills

Signs and symptoms:

  • Ulcers (pinpoint or extensive)

  • lethargy, lack of appetite

  • increased mucus on the gills or body

  • abnormal swimming

A common sign of bacteremia is hemorrhage at the base of the fins, along the top and lateral sides of the head and around the operculum and cloaca.

Treatments: Appropriate antibiotics in food, water or intramuscularly.

When choosing an appropriate antibiotic to fight infections in fish, keep in mind that most bacterial pathogens of fish are gram-negative.

  •  Improperly called Fish T.B.

  •  Probably the most frequently seen disease in home aquarium fish

  •  All species are susceptible

  •  Found in the water

  •  Squash of liver, spleen or blood (acid-fast stain for quick diagnosis)

  •  Culture can take up to 1 month

  • Treatment not really advisable and unlikely to be successful

 Zoonotic potential. Always wear gloves when handling fish or cleaning an aquarium. Fungal Diseases

Usually true opportunistic invaders. A check for underlying stressors will usually show another primary problem.

Most commonly encountered:

Saprolegnia spp - Common secondary infection to primarily dermal lesions or can act as a primary pathogen infecting fish that have not shown signs of previous damage. It is believed that primary pathogen episodes are temperature-dependant, usually occurring at low temperatures, possibly as a consequence of a reduced immune response..

Saprolegnia infection in a fish
Saprolegnia infection in a fish

Diagnose with skin scraping and gill clip.

Signs and symptoms: The organism looks like cotton wool to the naked eye.

Identification of both the causative agent as well as the nature of the predisposing condition are both important in disease control and long-term prevention.


  •  Itraconazole

  •  Formalin

  •  Malachite green

  •  NaCl

5.4. Common Problems Encountered When Treating Fish

5.4.1. The Pathogen

Not all ectoparasites are easily eliminated with one dose of drug (e.g., white spot requires repeated treatments). Prolonged treatments may not be done as directed or water changes not done as needed.

Drugs can be difficult to obtain (e.g., some ectoparasites might require the use of organophosphates).

Not all bacteria can be treated easily (e.g., Mycobacteria sp.) and therefore culling might be indicated.

Resistance to antibiotics is a significant factor in the treatment of bacterial diseases of fish. The liberal and often inappropriate use of antibiotics by hobbyist exposes the entire system to a multitude of antimicrobials, often leaving the commonly selected antibiotics ineffective.

Culture and sensitivity tests may take so long to complete it is often essential to start treatment as soon as possible, even prior to obtaining these results in order to avoid a disease "explosion".

5.4.2. The Patient

Size: Small fish and fry may be too small to inject safely.

Severe illness: Too ill (e.g., gill disease) to inject or handle without causing serious stress.

Unpalatable drugs (e.g., some antibiotics) and anorexic fish limit the use of in-feed medications.

Shoaling: Some fish (e.g., koi) prefer company particularly in isolation facilities, even if it is with other species.

Moribund fish
Moribund fish

6. Fish Emergencies

Emergency situations are a common scenario for the 'fish' practitioner. As with any other species, immediate action should be taken to stabilize the patient before any attempt is made to correct underlying problems.

A common scenario presented to veterinarians is the concerned owner of an aquarium or fish pond calling for advice after a few fish have suddenly died. As with other species, a diagnosis cannot be made over the phone, but initial supportive therapy can be begun to try to prevent more losses. The following steps provide a rough guide for the owners to follow:

  1. Immediately isolate any sick animals, and remove any dead individuals. Parasites and other pathogens will quickly leave the dead host and seek out live hosts spreading the disease throughout the entire population.

  2. Ensure adequate aeration in the system. Some aquatic systems can normally be maintained with minimal aeration; however, diseased fish have an increased need for oxygen, and just as with critically ill terrestrial or avian patients, they should be kept in an enriched oxygen environment. Remember that gas exchange happens at the surface of the water, so it is important to have a large surface area and to keep the surface water moving.

  3. Check the filtration system. Make sure that both the mechanical and chemical properties of the filtration system are working. In pond settings filters sometimes get clogged with a thick layer of mud, allowing the water to bypass the different filtration steps. As a result the water simply trickles back to the system unchanged. In an aquarium setting, make sure that the activated charcoal in the filter is not expired and that the filter floss has been cleaned or changed.

  4. Stop all feeding. The sick fish will not eat, and the leftover food will sink, decompose, and result in increased ammonia levels, creating a more toxic environment.

  5. Change one-third to one-half of the water. Use de-chlorinated water, or water treated with water conditioner, to dilute any toxic substances present in the fish's environment. Complete water changes are not recommended, as such drastic changes of the environment could become an additional stress factor and possibly kill the sick fish.

  6. Check water quality immediately. Levels of ammonia, nitrite, nitrate, pH are most important. Even if previous testing has been done, a repeat of the test is still advisable to rule out any errors and to compare the water quality after the partial water change.

  7. If no salt has been added to the system before, add non-iodized salt at 2 g/L. This quantity equals a 0.2% solution. For example a 10 gal system would need about 80 g. of salt!! Initially add 50% of the calculated value of salt, wait 24 hours, and then check the water salinity level. If it is at the appropriate level, add the rest of the calculated value. Kosher table salt is non-iodized, readily available, and cheap. Salt will help to improve the osmotic balance between freshwater fish and their environment and therefore reduce stress on the fish's metabolic system. Salt added at this concentration will not harm the fish in any way. In addition it may also kill many ectoparasites and will block the uptake of toxic ammonia by the gills. The only negative side-effect is that this concentration might possibly kill some freshwater plants. Also before adding salt make sure that the charcoal in the filter does not contain zeolite, as adding salt will release ammonia bound to this filter medium.

6.1. Further Investigation

Once the system has been initially stabilized, attempts should be made to prevent ongoing losses. Good communication with the owner is a key to success in diagnosing the underlying problem. It is a good practice to have a standard questionnaire prepared which can be filled out by the owner while first aid is being initiated. In questioning the owner it is extremely important to get a feeling for their understanding of fish medicine. It is a common scenario that multiple antibiotics and antiparasitics have already been applied to the system by the owner, and therefore that the pathogens present might already exhibit multi-drug resistance. Questions such as these establish a minimum database from which to pursue further diagnostics:

  •  How long have you been keeping fish?

  •  When did you first notice these problems?

  •  How long have you owned the sick fish?

  •  Are there other fish in the same tank or pond with the sick fish, and if so, how are they now?

  •  What is the size (volume) of the tank or pond and how is it heated, filtered, and aerated?

  •  What and how often do you feed your fish?

  •  Have the fish already been treated? If so, by whom and with what medications?

  •  Is there a possibility that the fish were exposed to some type of toxin?

7. Common Scenarios

7.1. Abnormal water chemistry parameters

This is the number one fish killer. Careful monitoring of these parameters through regular water checks and corrective water changes can save the life of many fish

7.1.1. Suffocation

This usually originates from an increase in water temperature which results in a decrease in the oxygen available in the system (or the air was accidently turned off).

7.1.2. Chlorine intoxication

The use of chlorinated tap water without pretreatment with dechlorinating agents can create lethal chlorine levels which can kill all fish within hours. It is important to note that simply aging tap water will not effectively make it safe to use if chloramine complexes are present since they will not evaporate out of water the way chlorine will and will remain at harmful levels.

7.2. Tank mate aggression

Fish are very territorial and aggressive individuals can prevent other fish from eating or even inflict actual wounds. High stress levels in the subordinate fish can increase their susceptibility to disease.

8. Emergency Drugs

8.1. Hydrogen peroxide (H2O2)

As water temperature increases, the concentration of dissolved oxygen decreases. If fish show signs of suffocation, through piping behavior, an increase in the dissolved oxygen concentration is needed.

Use commercially available H2O2 (3% solution) at a rate of 24-25 ml per 10 gal of water. Caution: It is a very caustic agent in the concentrated form, and the gills can be damaged by direct contact.

8.2. Ice Packs

These are a great way to cool down water temperatures, and thereby increase the oxygen carrying capacity, of the water. Owners of ponds should always have a few ice packs in the freezer just for this purpose.

8.3. Sodium bicarbonate (Baking powder)

When a system crashes the pH often falls significantly. Sodium bicarbonate will act as a buffer and prevent the pH dropping too low. One teaspoon per 10 gal. water should be enough.

8.4. Dexamethasone

This should only be used in the shocky or traumatized patient at a dose of 1-2 mg/kg.

9. Prevention

The best way to deal with emergencies is simply to avoid them. Make sure that you have educated your fish client appropriately right from the initial visit. This education can be accomplished even if the first aid attempts were unsuccessful and the patient died. Most clients will learn from this experience, and are willing to be educated as to how to prevent further losses and avoid future die offs. It is important to discourage the use of poly-pharmacy by the lay person thereby increasing the chances of a controlled application of drugs to succeed. Additionally one of the most frustrating experiences is the outbreak of disease in an established system due to the introduction of a new pathogen through improper quarantine protocols. Often once a system has been managed successfully without major losses or complications, owners forget about all the different problems that could be encountered via new acquisitions. Make sure your clients are reminded on a frequent basis about this issue and educated to practice proper quarantine when adding new individuals to their previously closed colony.

10. References and Resources

Conservation Medicine Challenges

Supplemental Readings


10.1. Professional Organizations

International Association for Aquatic Animal Medicine

World Aquatic Veterinary Medical Association .

American Veterinary Medical Association

10.2. Web Sites

Aquaculture Dictionary - Aquatext

Fishdoc... - great site for disease identification (pictures and movies) - Info on diseases but more of a PR for the book and the video.

Freshwater Aquariums... - Good info, site is monitored by host, hence reliability of info better

Saltwater Aquariums...

Food Animal Residue Avoidance Databank

10.3. Water test kits

Gilford Instrument Labs, Inc.Ovelin, OH 44074 ; 216-774-1041 .

Hach Company P.O. Box 389, Loveland, CO 80539 ; 800-227-4224

La Motte Chemicals. P.O. Box 329, Chestertown, MD 21260 ; 301-778-3100

10.4. Texts and Articles

BSAVA Manual of Ornamental Fish, 2nd Ed. Edited by William H. Wildgoose, UK- BSAVA, 2001.

Brown, Lydia. Aquaculture for Veterinarians: Fish husbandry and Medicine. Pergamon Press.

Browser P.R. Anesthetic options for fish, IN: Recent advances in Veterinary Anesthesia and Analgesia: Companion animals, R.D. Gleed, J.S. Ludders (eds.) IVIS website

Fowler, M. and R. Miller eds. Zoo and Wild Animal Medicine. Current Therapy 4. 1997. Chapters 23, 24, 25.

Ferguson, H. 1989. Systemic Pathology of Fish. Iowa State University Press. Ames.

Food and Drug Administration Center for Veterinary Medicine. Judicious Use of Antimicrobials in Aquatic Animals , 2006.

Gratzek, John et al. Aquariology. The Science of Fish Health Management.

Lewbart, G. Emergency Pet Fish Medicine. 1998. Vet Clinics of North America: Exotic Animal Practice. Vol. 1(1) pp. 233-249.

Lewbart, Gregory A., et al. Evaluation of a method of intracoelomic catheterization in koi. JAVMA, Volume 226 (5), 2005 : 784-788.

Mayer, F. and Lee Barclay Eds. 1990. Field Manual for the Investigation of Fish Kills. United States Department of the Interior, Fish and Wildlife Service / Resource Publication 177

Friend, M. Ed. Field Guide to Wildlife Diseases. United States Department of the Interior, Fish and Wildlife Service / Resource Publication 167

Noga, E. Fish Disease: Diagnosis and Treatment. Mosby-Year, Inc. St. Louis. 1996. ISBN 1-55664-374-8.

Ostrander, G.K. Ed. 2000. The Laboratory Fish. Bath Press, Bath, Somerset, UK.

Sindermann, C. 1990. Principal Diseases of Marine Fish and Shellfish. Vols. 1 and 2, Academic Press, Inc. San Diego.

Smith, Stephen A. Nonlethal clinical techniques used in the diagnosis of diseases of fish. JAVMA , v.220(8), 2002: 1203-1206.

Stoskopf, M. Fish Medicine. Saunders. 1993.

Yanong, Roy P.E., et al. Algal dermatitis in cichlids. JAVMA, v.220(9), 2002: 1353-1358.