Fish Medicine
American Association of Zoo Veterinarians Conference 1999
Ruth Francis-Floyd, DVM, MS, DACZM
Department of Fisheries & Aquatic Sciences, University of Florida, Gainesville, FL, USA


The essentials of fish medicine can be divided into a few basic areas; water quality, preventive medicine, parasite control and control of bacterial infections. Each of these areas needs to be addressed to achieve a successful fish health medicine program. In most situations, water quality management and preventive medicine are more important, and rewarding, than treatment of infectious disease.

Water Quality (It's the Environment)

Water quality management is the component of fish medicine that is often most foreign to veterinarians. The importance of mastering a basic understanding of this area, as it relates to animal health, cannot be overemphasized. The veterinarian must understand water quality well enough to interact effectively with environmental engineers and fisheries biologists or aquarists. Veterinarians can and should be a vital member of the water quality team in a zoo setting.

For day-to-day fish health management there are only a few basic parameters that need to be mastered. Some of these have a direct impact on animal health (e.g., dissolved oxygen), others may indirectly affect animal health because of interaction with another component of the water (e.g., ammonia and pH), or because of the effect on a drug or chemical (e.g., total alkalinity and copper toxicity).

The basic water quality parameters for freshwater systems are: dissolved oxygen, temperature, pH, ammonia, nitrite, chloride, total alkalinity, and total hardness. In marine systems, nitrate and salinity should also be monitored on a regular basis. In addition, some parameters may be of special concern in certain types of systems. For example, a system using city water as it’s source water should be tested for chlorine and/or chloramine. A system using a deep well for source water should be evaluated for nitrogen gas and carbon dioxide to avoid potential problems with “gas bubble disease.” Heavy metals are occasionally of concern, older facilities with copper piping can be disastrous for fish. Zinc will leach from galvanized metals that are located somewhere on the system, and hydrogen sulfide or iron can be problematic. Every effort should be made to thoroughly understand the water being used to set up a system, the treatment of the water on the system, and the essentials of the engineering such as flow rates, turnover rates, filter construction and capacity. Initially this can be tiresome or intimidating but it is essential to develop this expertise to successfully manage aquatic species.

Dissolved Oxygen

Dissolved oxygen is simply oxygen gas in solution in water. Most fish prefer ≥5 mg/L for optimal health, and coldwater species often have a higher oxygen requirement than warmwater species. Clinical distress often becomes apparent at dissolved oxygen concentrations of 2–4 mg/L, depending on species, and mortalities usually occur when levels drop to ≤1–2 mg/L. Some species are more sensitive and may die at significantly higher concentrations, for example, hybrid striped bass are severely stressed and may start to die at dissolved oxygen concentrations of 4 mg/L. When an oxygen depletion occurs large fish are usually more severely affected than small ones. There are also important species-specific differences in tolerance for low dissolved oxygen.

In outdoor systems in which algae is allowed to grow, there is a diurnal oxygen cycle driven by photosynthesis. Dissolved oxygen is lowest at dawn, following respiration all night long by fish and other biota in pond. Dissolved oxygen is highest late in the afternoon, following maximum exposure of the water surface to sunlight. Anything that decreases the intensity of sunlight hitting the surface of the pond, such as cloudy weather, can significantly decrease the amount of oxygen produced. The “greener” the water, the more extreme the diurnal fluctuation; therefore, management of “blooms” becomes very important. Algal blooms an be monitored by measuring the visibility of the water with a “secchi disc”. Any time visibility is less than 18 inches the danger of a catastrophic oxygen depletion caused by an excessive bloom is significant. Algal blooms die, turning the water brownish, gray, or even black. The visible color change often precedes an oxygen depletion; therefore, managers or keepers should be educated to pay attention to the appearance of water.

Certain physical properties of water and oxygen are important for the clinician to keep in mind. First, oxygen concentration (at saturation) increases as temperature, salinity and altitude decrease. Therefore, hot water is able to hold less oxygen in solution—hence, the increase in oxygen depletions in summer. Salt water is also able to hold less oxygen than freshwater at the same temperature; however, the influence of salinity (or altitude) is far less significant than the influence of temperature.


Fish are poikilothermic, therefore environmental temperature has a huge influence on metabolism. Each species has a preferred temperature range, some are very tolerant of temperature fluctuation, others are not. Temperature has a huge influence on the immune system of fish, changes of 5°C have been shown to shut down T-cell function of channel catfish for 3 wk, and to transiently slow the specific immune response to the point where it is essentially non-functional. Many infectious diseases have a "preferred" temperature window, therefore manipulation of environmental temperature is a legitimate therapeutic strategy for some problems.


pH chemistry in water is complex and controlled, in part, by the carbonate cycle. Hydrogen ions complex with carbonates to form bicarbonates. The total alkalinity is a measure of how much carbonate is present, and therefore is closely tied to pH. Any time pH of water is inappropriate the carbonate concentration, or alkalinity, should be checked. This is also discussed below in the section on alkalinity.

For freshwater fish, pH in the range of 6.5–9.0 is a good “normal” range. Some fish, such as Amazon species, prefer more acidic water, while others, such as the Rift Lake cichlids from Africa, prefer more alkaline conditions. In general, pH of 4.0 and 11.0 are lethal to most freshwater species. pH outside the “normal” range may not be lethal but lead to “stress”, reflected in poor growth and reproduction. For marine species, pH of 8.2–8.3 is considered “normal.” The marine environment is very stable, and these animals are much less tolerant of inappropriate pH. A pH less than 7.5–7.7 or greater than 8.3–8.5 can be stressful for many marine species.

In “green water,” such as outdoor ponds with an algal bloom, carbon dioxide fluctuates daily, as does oxygen. Carbon dioxide provides the carbon source for plants during photosynthesis, therefore the concentration decreases dramatically during the day, resulting in a rise in pH. At night, carbon dioxide accumulates because respiration is occurring in the absence of photosynthesis, and as carbon dioxide increases the pH falls. These changes are not only important because of their impact on fish health, but also because of the impact on other chemicals in the water, particularly ammonia (see below).


The most important source of ammonia in aquatic systems is usually fish food because it is very high in protein. In an established system there is an ammonia cycle that removes nitrogen and recycles it. The first two steps in the process are aerobic and only occur in the presence of oxygen. Nitrosomonas bacteria oxidize ammonia to nitrite, and Nitrobacter oxidize nitrite to nitrate. Under anaerobic conditions, nitrate can be reduced to nitrogen gas. That is then released to the atmosphere. Nitrate can also be used directly by plants.

In aquatic systems, ammonia exists in two forms, ammonia and ammonium. Ammonia (NH3) is highly toxic and concentrations as low as 0.05 mg/L can cause gill damage, while concentrations of 2.0 mg/L are often lethal. Ammonium (NH +) is sometimes considered “non-toxic”, but “less toxic” is probably a better way to consider it. The form of ammonia present in water is controlled by pH, the more acidic the water, the greater the concentration of ammonia.

Acute ammonia toxicity is often associated with neurologic signs including spinning and convulsing. Chronic toxicity may result in renal compromise and opportunistic infections. Aminoglycosides are renal toxic and this may be exacerbated by high ammonia conditions. The best short-term treatment for an ammonia problem is a water change. It is also important to determine why the ammonia has spiked and to correct any underlying problems. The most common causes of ammonia accumulation are overfeeding, biofilter failure, or phytoplankton die-off.


The second breakdown product in the nitrogen cycle is nitrite. Nitrite causes methemoglobinemia and results in hypoxia. This condition is often referred to as “brown blood disease” because the gills and blood become chocolate brown in color as the percentage of hemoglobin present as methemoglobin increases to about 80%. There are species-specific sensitivities to nitrite toxicity, with channel catfish and freshwater angelfish being extremely sensitive, while centrarchids (bass and bluegill) are refractory.

The treatment for nitrite toxicity of freshwater fish is chloride, usually delivered as sodium chloride. The chloride molecule has approximately the same size and ionic charge as nitrite. Increasing the chloride concentration to at least six times the nitrite concentration sets up a competitive inhibition at the epithelial surface of the gills. The result is a decrease in the amount of nitrite crossing into the blood and a consequent reduction in the amount of methemoglobin that forms. Fish can recover from active disease within 24 hr when treated with chloride.

Chronic nitrite exposure has been associated with development of anemia in channel catfish. It is assumed that the lifespan of the red blood cell is shortened due to the constant need to reduce methemoglobin, using up cell's energy reserves. Chronic nitrite toxicity can be difficult to diagnose unless excellent water quality records are available for several weeks preceding the suspect event.


Nitrate is the final product of the aerobic part of the nitrogen cycle. Nitrate is rarely a problem in freshwater systems because water changes flush the ion from the water, reducing the concentration. In marine systems, however, where water changes either are not done at all, or are rare, nitrate can accumulate to several hundred mg/L. The exact impact of this on fish health is poorly understood, but there are some species that do not seem to do well in high nitrate environments.

Nitrate can be removed from closed marine systems by an anaerobic process. There seem to be limits to this technique but it is being used more commonly in large marine systems for which water changes are not possible.

Total Alkalinity

As mentioned above, total alkalinity is a measure of the carbonate concentration (buffering capacity) of water. For freshwater systems, total alkalinity should be increased if it is below 50–100 mg/L for most species. This can be done using agricultural limestone (calcium carbonate) in large bodies of water (>0.25 surface acres) but dolomite (calcium and magnesium carbonate) is often easier to use in smaller systems.

Marine systems should have very high alkalinity (>250 mg/L). Crushed coral or limestone can be used to raise and maintain high alkalinity in these conditions.

When the alkalinity is very low a pH crash can occur as organic acids accumulate in the system. Anytime excessively low pH is detected, low alkalinity should be suspect.

Copper sulfate toxicity is directly related to the total alkalinity of the water. If total alkalinity is below 50 mg/L copper sulfate cannot be used safely in freshwater systems. Copper sulfate dose can be titrated with total alkalinity for safe use of this compound in freshwater (total alkalinity % 100; dose not to exceed 2.5 mg/L).

Total Hardness

Total hardness measures the divalent cations in water. These include calcium, magnesium, manganese, iron and zinc. Frequently, the total hardness and total alkalinity are about the same value as carbonates usually form ionic bonds with calcium and magnesium.

The total hardness of water is very important for rearing fry as fish absorb minerals from water for bone development and other metabolic processes. Calcium concentrations <20 mg/L have been demonstrated to be very detrimental to channel catfish fry. The calcium concentration of water can be increased by adding calcium chloride.


For freshwater fish a chloride concentration of 100 mg/L is recommended to prevent nitrite toxicity in susceptible species. Chloride and other ions should be avoided in tanks housing fish in the family Morimidae (e.g., elephant nose, black ghost, etc.). These animals are from very soft Amazon water and navigate by creating a small electrical current. Excessive ions in the water totally disrupt this process.


Seawater is a 3% salt solution, which is the same as approximately 30 ppt or 30,000 ppm. Salinity of 0.02% (200 ppm) may be enough to prevent or decrease some protozoal infections of freshwater fish. Salinity of 5 ppt (0.5% or 5,000 ppm) is good for short-term transport (hours to days) of most freshwater fish. This concentration is also an excellent anti-protozoal treatment.


Chlorine and chloramine are commonly used in municipal water supplies to eliminate bacteria. Chloramine is a combination of chlorine and ammonia. When the chlorine is removed, the ammonia remains. Chlorine concentrations of 0.02 mg/L are toxic to fish and concentrations of 0.2 mg/L are lethal to many species.

Chlorine can be removed from water using sodium thiosulfate. For every 1 mg/L of chlorine present, 7.4 mg/L sodium thiosulfate can be added to remove it. Sodium thiosulfate can remove oxygen from water so excellent aeration is necessary when using dechlorinators.

Gas Bubble Disease

“Gas bubble disease” is usually caused by supersaturation of water with nitrogen gas. This is most common when water is brought up from a deep well. Excess gas can be eliminated by spraying water before it comes into contact with fish. A pathognomonic lesion for gas bubble disease is gas emboli within gill capillaries. These can be seen on gill biopsy.

Control of Protozoal Diseases

There are four basic chemicals used to control protozoal diseases of fish: formalin, copper sulfate, potassium permanganate and salt. Selection of one of these agents is often based on constraints imposed by the system, rather than differences in efficacy.


Formalin is FDA approved as an ectoparasiticide for fish. FDA-labeled formalin products contain 37% formaldehyde gas in aqueous solution, and methanol may be added as a preservative. Product that appears cloudy or has a white precipitate may contain paraformaldehyde, a highly toxic substance that forms when formalin gets cold (<45°F).

Formalin concentrations of 12–25 mg/L (1–2 drops per gallon) are effective as an indefinite bath. The concentration can be increased to 170–250 mg/L for 30 min but it is a harsh treatment. Formalin usually has excellent efficacy against protozoans, and moderate efficacy against monogeneans, columnaris bacteria, and saprolegnia. It chemically removes oxygen from water so excellent aeration during treatment is important.

Copper Sulfate

Copper sulfate is EPA approved as an algicide. It is also effective as an anti-protozoal agent, however FDA approval for this use has never been issued. It is extremely toxic to fish and even more toxic to invertebrates and plants. Never use copper in a system with plants or invertebrates that you like!

For freshwater fish the concentration of copper sulfate can be titrated with the total alkalinity of the water. If total alkalinity is <50 mg/L do not use copper. If the total alkalinity is 50–250 mg/L, the concentration of copper sulfate is equal to 1% of the total alkalinity. For example, if the total alkalinity is 100 mg/L, then the concentration of copper sulfate that can be used safely and effectively is 1 mg/L. Never use more than 2.5 mg/L copper sulfate, regardless of how high the alkalinity is.

For marine systems, slowly bring the copper concentration (not copper sulfate concentration) up to 0.02 mg/L and try to hold it there for 3 wk. Copper concentration will need to be monitored closely (1–2 times/day) and additional chemical added as needed.

Copper solutions are extremely effective against protozoal infections and moderate efficacy against monogeneans, columnaris and Saprolegnia.

Potassium Permanganate

Potassium permanganate does not have FDA approval for aquaculture use but is used routinely in water softening systems. It functions as an oxidizing agent and essentially sanitizes the external surface of fish, removing parasites, fungus and bacteria. It works very well with salt in small tanks.

Potassium permanganate is primarily used on freshwater fish, and there is very little information on safety for marine fish. It can cause significant gill damage, especially when used repeatedly. A safe recommendation is no more than one treatment per week.

Chemical concentration is usually 2 mg/L as an indefinite bath, or 10 mg/L as short-term bath (30 min). The chemical will oxidize whatever organic matter is present and therefore the concentration must be increased when used in organically rich environments.


Salt is mentioned briefly above. It is a wonderful anti-protozoal treatment for many freshwater fish. As mentioned above, most freshwater fish will tolerate a mild to moderate increase in salinity for a fairly long period of time. Freshwater fish probably tolerate an increase in salinity better than marine fish tolerate a decrease in salinity.

Control of Crustacean Parasites

Difluorobenzuron (Dimilin)

Difluorobenzuron has recently been approved by EPA for use on aquatic sites, however a Restricted Use Pesticide License is required to apply the compound. It inhibits chitin synthesis and is an excellent product for treatment of anchor worms and other crustacean parasites of fish. The chemical should not be used in water with an effluent due to its prolonged half-life (no effluent for at least 14 days following treatment). The compound should be used at a concentration of 0.03 mg/L as a prolonged bath.

Control of Monogenetic Trematodes


Praziquantel has no approval for use in aquatic sites or on aquatic species. It is effective, however in controlling monogenetic trematodes and intestinal tapeworms, and is most commonly used in closed marine systems. Concentrations as low as 2 mg/L have been effective in controlling monogeneans in marine fish and the chemical may remain active for 2 wk, killing juvenile forms that hatch after the initial chemical application.

Control of Internal Parasites


Metronidazole is not approved for use in any aquatic species or site, however it has excellent efficacy against intestinal flagellates found in ornamental fish. It can be delivered in a medicated feed at a dosage of 50 mg/kg body weight (4.5 g/lb food). Fish should be fed the medicated feed for five days. If fish are not eating it can be delivered as a bath treatment at a concentration of 6 mg/L (250 mg/10 gal water), repeated daily for 5 days. A water change (50%) 4–8 hr after treatment is recommended to remove residue.


Fenbendazole has not been approved for use on any aquatic species or for any aquatic site. Some efficacy has been demonstrated against intestinal nematodes when given orally, however there is more anecdotal information than actual efficacy data. The compound can be fed at a dose of 3.5 g/lb food for 3 days, and repeated in 3 wk.

Antibiotics for Fish

Terramycin (Oxytetracycline)

Terramycin is FDA approved for use in salmonids, channel catfish, and lobsters. It is a broad-spectrum antibiotic, effective against many gram-negative organisms, although resistance to terramycin is common. The dose is 55 mg/kg body weight, and it is usually sold as manufactured medicated feed. The drug should be fed for 10 days, and in food species, followed by a 21-day withdrawal period. Terramycin is sold as a sinking medicated feed as oxytetracycline is destroyed by heat required to process pellets for floating.


Romet is a potentiated sulfa containing ormetoprim and sulfadimethoxine. It has been approved by FDA for use in salmonids and channel catfish. The dose is 50 mg/kg body weight and it is usually sold as pre-manufactured medicated food. The drug should be fed for 5 days. It has a 3-day withdrawal time in channel catfish and a 6-wk withdrawal period for salmonids. Romet is heat stable and therefore is available in floating pelleted rations.


Erythromycin has not been approved for use in food animals but is generally considered the treatment of choice for infections by gram-positive organisms. It can be fed at a dose of 150 mg/kg body weight for 14 days. It has also been delivered by i.m. injection to salmonids to prevent infection by Renibacterium salmoninarum (bacterial kidney disease agent).


Aminoglycosides have not been approved for use in any aquatic species, however they are used by koi enthusiasts to treat gram-negative infections, particularly Aeromonas salmonicida. These compounds are renal toxic. Gentamicin has been injected into goldfish to create a biomedical model for polycystic kidney disease—Not recommended when ammonia levels are elevated.


MS-222 (Methane Tricaine Sulfonate)

Methane tricaine sulfonate has been approved by FDA as an anesthetic for use in fish. It is usually delivered at concentrations of 50–200 mg/L. Essentially, the compound should be used to effect, and opercular beats carefully monitored. Aeration is important during any anesthetic procedure, and if the fish is to be anesthetized for a period of time the concentration should be reduced following induction. MS-222 has a 21-day withdrawal period when used on food species.


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

Ruth Francis-Floyd, DVM, MS, DACZM
Department of Fisheries & Aquatic Sciences
University of Florida
Gainesville, FL, USA

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