The most obvious sign of sick fish is the presence of dead or dying animals. However, the careful observer can usually tell that fish are sick before they start dying because sick fish often stop feeding and may appear lethargic. Healthy fish should eat aggressively if fed at regularly scheduled times. Pond fish should not be visible except at feeding time. Fish that are observed hanging listlessly in shallow water, gasping at the surface, or rubbing against objects indicate something may be wrong. These behavioral abnormalities indicate that the fish are not feeling well or that something is irritating them. In addition to behavioral changes, there are physical signs that should alert producers to potential disease problems in their fish. These include the presence of sores (ulcers or hemorrhages), ragged fins, or abnormal body confirmation (ie, a distended abdomen or "dropsy" and exophthalmia or "popeye"). When these abnormalities are observed, the fish should be evaluated for parasitic or bacterial infections.

If you suspect that fish are getting sick, the first thing to do is check the water quality. If you do not have a water quality test kit, contact your resident aquatic veterinarians; Anyone contemplating commercial production of fish should invest in a water quality test kit and learn how to use it. Low oxygen is a frequent cause of fish mortality in ponds, especially in the dry season. High levels of ammonia are also commonly associated with disease outbreaks when fish are crowded in ponds, vats, or tanks. In general, check dissolved oxygen, ammonia, nitrite, and pH, during a minimum water quality screen associated with a fish disease outbreak. Ideally, daily records should be available for immediate reference when a fish disease outbreak occurs. These should include the dates fish were stocked, size of fish at stocking, source of fish, feeding rate, growth rate, daily mortality and water quality. This information is needed by the resident veterinarian working with you to solve your fish disease problem. Good records, a description of behavioral and physical signs exhibited by sick fish, and results of water quality tests provide a complete case history for the vets working on your case.

There are two broad categories of disease that affect fish, infectious and non-infectious diseases. Infectious diseases are caused by pathogenic organisms present in the environment or carried by other fish. They are contagious diseases, and some type of treatment may be necessary to control the disease outbreak. In contrast, non-infectious diseases are caused by environmental problems, nutritional deficiencies, or genetic anomalies; they are not contagious and usually cannot be cured by medications. Infectious diseases Infectious diseases are broadly categorised as parasitic, bacterial, viral, or fungal diseases. Parasitic diseases of fish are most frequently caused by small microscopic organisms called protozoa which live in the aquatic environment. There are a variety of protozoans which infest the gills and skin of fish causing irritation, weight loss, and eventually death. Most protozoan infections are relatively easy to control using standard fishery chemicals such as copper sulfate, formalin, or potassium permanganate. Bacterial diseases are often internal infections and require treatment with medicated feeds containing antibiotics which are approved for use in fish by the Food and Drug Administration. Typically fish infected with a bacterial disease will have haemorrhagic spots or ulcers along the body wall and around the eyes and mouth. They may also have an enlarged, fluid-filled abdomen, and protruding eyes. Bacterial diseases can also be external, resulting in erosion of skin and ulceration. Columnaris is an example of an external bacterial infection which may be caused by rough handling. Viral diseases are impossible to distinguish from bacterial diseases without special laboratory tests. They are difficult to diagnose and there are no specific medications available to cure viral infections of fish. Consultation with your resident vet is recommended if you suspect a bacterial or viral disease is killing your fish. Fungal diseases are the fourth type of infectious disease. Fungal spores are common in the aquatic environment, but do not usually cause disease in healthy fish. When fish are infected with an external parasite, bacterial infection, or injured by handling, the fungi can colonise damaged tissue on the exterior of the fish. Formalin or potassium permanganate are effective against most fungal infections. Since fungi are usually a secondary problem it is important to diagnose the original problem and correct it as well. Non-infectious diseases Non-infectious diseases can be broadly categorised as environmental, nutritional, or genetic. Environmental diseases are the most important in commercial aquaculture. Environmental diseases include low dissolved oxygen, high ammonia, high nitrite or natural or man-made toxins in the aquatic environment. Proper techniques of managing water quality will enable producers to prevent most environmental diseases. Nutritional diseases can be very difficult to diagnose. A classic example of a nutritional disease of catfish is "broken back disease," caused by vitamin C deficiency. The lack of dietary vitamin C contributes to improper bone development, resulting in deformation of the spinal column. Another important nutritional disease of catfish is "no blood disease" which may be related to a folic acid deficiency. Affected fish become anaemic and may die. The condition seems to disappear when the deficient feed is discarded and a new feed provided. Genetic abnormalities include conformational oddities such as lack of a tail or presence of an extra tail. Most of these are of minimal significance; however, it is important to bring in unrelated fish for use as broodstock every few years to minimise inbreeding.

The aquatic environment provides a unique and effective means for the transmission of pathogens. Maintenance of suitable water quality greatly reduces the various stressors to which fish are exposed, which in turn reduces the likelihood of a disease problem. Critical water quality parameters include temperature (particularly sudden and dramatic shifts), dissolved oxygen, pH, alkalinity, hardness, nitrogenous wastes (un-ionized ammonia (NH3)); nitrite (NO2- anion), and potentially toxic substances (e.g., heavy metals, pesticides, carbon dioxide (CO2)). Water quality should be monitored frequently and corrective measures initiated if conditions become stressful. The frequency of water quality monitoring will differ with different types of production systems and with the specific parameter being monitored. Dissolved oxygen in a pond can vary by a considerable amount during the day and is often measured several times during a 24-hour period. Hardness and alkalinity will typically be stable in a pond system, while it will change, but over a period of months in a closed recirculation system. The use of high-quality feed provides fish with the nutrients that they need to remain healthy and grow rapidly. Fish which are fed a nutritionally complete diet are better able to cope with stress and resist disease. Fish farmers should remember that even high-quality feeds will deteriorate if improperly stored or kept too long. The feed should be purchased from a reputable supplier, stored in a cool and dry place, and used in a timely manner (typically within 90 days or longer per the manufacturer’s instructions). Light (excessive or rapid changes in intensity), noise, and other disturbances can stress fish and should be minimized. Routine maintenance, stocking, and harvesting require that fish be handled. When fish are removed and processed (e.g., weighed, sorted, transported) they compensate physiologically. To reduce the trauma of handling make sure all necessary materials (e.g., nets, hauling tanks, weighing scales) and adequate personnel are immediately available. Handle the fish gently and for as short a time as possible. If possible, do not handle fish that are already stressed or when environmental conditions are marginal (e.g., too hot, too cold, inhospitable).