Basic fish medicine (Proceedings)


The origins of common problems encountered in aquatic medicine are discussed, with emphasis on the importance of monitoring water quality. As with other species, the goal of fish medicine should be prevention and not treatment!

Objectives of the Presentation

The origins of common problems encountered in aquatic medicine are discussed, with emphasis on the importance of monitoring water quality. As with other species, the goal of fish medicine should be prevention and not treatment!

Key Points

     • Most problems in aquatic animal health originate from environmental factors.

     • The origins of common scenarios found in pet fish medicine are described and solutions discussed.

Overview of the Issue

      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.

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.

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, dropsy). 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.


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.

Common Husbandry Problems of Captive Fish

      1. Failure to know the natural history of the fish.

      2. Failure to let the system age (6 weeks) before adding fish.

      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.

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

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

      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. Failure to keep adequate records and failure to review the recorded data.

      13. Misinterpreting reproductive or other normal behavior as aberrant.

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

      15. Failure to provide proper substrates, shelter, or support.

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

      17. The belief that a few treatments with antibiotics (sometimes inappropriate) will solve all issues.

Common Infectious Diseases

For a good diagnostic work up, it is necessary to work with a living fish or a freshly killed specimen. A dead fish will only be good for histology, if that!!


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.

Diagnostic techniques for common parasites include:

     • Ich (Ichthyophthirius multifiliis): Skin scrape, gill clip

     • Trichodina spp.: Skin scrape, gill clip

     • Costia

     • Lernaea spp. (Anchor worm): Skin and gill exam

     • Argulus spp. (Fish lice): Skin exam

Animals infected with ectoparasites are often irritated by them and may therefore show abnormal behavior patterns (e.g., flashing, scraping)


     • Salt non-iodized

     • Potassium Permanganate

     • Ivermectin

     • Formalin etc. (See Lewbart, 2002)

Bacterial Diseases

Remember that fish live in a bacterial soup and are constantly challenged by the presence of these organisms. Most infections are opportunistic. A common sign is hemorrhage in the fins and scales Fin rot is a bacterial disease involving opportunistic bacteria such as Aeromonas, Pseudomonas or Flexibacter. It is often seen in newly acquired fish and is usually self-limiting, resolving as they settle in. Fin erosion demonstrates just how sensitive fish can be to stress, and its presence is often a sign that all is not well.


     • Improperly called Fish T.B.

     • Probably the most frequently seen disease

     • All species are susceptible

     • Found in the water

     • Squash of liver, spleen-acid-fast stain

     • Culture can take up to 1 month

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

Treatment not really advisable and unlikely to be successful

Other common bacteria

     • Aeromonas hydrophila: Culture of blood, liver, kidney

     • Vibrio spp: Culture of blood, liver, kidney

     • Edwardsiella: Culture of deep ulcers, blood

     • Columnaris: Squash prep of gills


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

Saprolegnia can act as a primary pathogen infecting fish that have not shown signs of previous damage. It is believed that such episodes are temperature-dependant, usually occurring at low temperatures, possibly as a consequence of a reduced immune response.

Diagnose with skin scraping and gill clip. 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

Common Problems Encountered When Treating Fish

      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.

Bacterial resistance to antibiotics is a significant factor in the treatment of bacteria 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".

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.


Much more than with terrestrial patients, environmental conditions influence the dynamics of pathological processes. This is due to two major factors. First, as a poikilotherm, the fish and all its metabolic processes are governed by environmental temperature. Second, the fish and the environment (water) are much more closely connected to each other than is seen with terrestrial animals and their environment. Controlling and managing environmental factors is the KEY to success in aquatic medicine. Without optimal environmental conditions, no treatment will be truly successful.


Wildgoose, WH: BSAVA Manual of Ornamental Fish 2nd ed. Iowa State University Press, ISBN: 0905214579 ($140),

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