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Disorders and Diseases of Fish


Ruth Francis-Floyd

, DVM, DACZM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida;

Barbara D. Petty

, DVM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida

Reviewed/Revised Feb 2020 | Modified Oct 2022
Topic Resources

Many of the disorders and diseases that are known to occur in fish are the result of stress, poor water quality, overcrowding, and failure to quarantine any new or sick fish to avoid spread of disease. These factors can all be minimized by appropriate care and good hygiene. Infections caused by bacteria, protozoa, viruses, fungi, or parasites may also occur.

Heart and Blood Vessel Disorders

Fish are cold-blooded, which means their body temperature is close to and fluctuates with the temperature of their environment. In addition, all their bodily processes are greatly influenced by the water temperature. Water that is very cold or that has been under pressure can become supersaturated with dissolved gases. If the temperature rises or the pressure drops suddenly, these gases may expand rapidly. If fish have already been exposed to this supersaturated water, the gases they absorbed while breathing may also expand rapidly, releasing gas into the bloodstream. This is called gas bubble disease, and the small bubbles created can result in much tissue damage and death. Gas bubble disease in pond fish can be caused by owners filling an outdoor pond with well water using a hose. If the hose is submerged, gas in the incoming water will stay dissolved in the water and can cause problems. This is especially important if the water source is a deep well. To prevent this, the inflowing water can be aerated by spraying it from above as it hits the tank or pool.

Excess ammonia in a system is very harmful to fish. High levels of ammonia in an aquarium can be caused by several factors. Two syndromes that are characterized by very high levels of ammonia are well described. The first is called new tank syndrome and is a simple accumulation of ammonia that occurs when a new tank is stocked with fish before the biological filter is fully functional. This syndrome usually occurs within the 1 to 3 weeks after a new system is set up. Ammonia accumulates because there are not enough bacteria in the biofilter to metabolize it. The situation can be managed by frequent (sometimes daily) water tests. When total ammonia levels are high (2 mg/L or higher), at least 50% of the water in the aquarium should be changed. This intense management should only be necessary for 1 to 2 weeks unless there are other unidentified problems with the system. Water quality monitoring will show a decline in ammonia, followed by an increase in nitrite as the bacterial colonies grow. The process is complete when nitrite levels also fall to normal.

The second type of ammonia problem is called old tank syndrome. Although it is also characterized by high ammonia concentrations, it is completely different. This problem is caused by a sudden and drastic drop in pH, often below 6.0, which kills bacteria in the biofilter. The loss of bacteria results in the high concentrations of ammonia, which are the hallmark of this problem. Simple water changes are not recommended, however, because an increase in pH may cause the ammonia present to become toxic, killing the fish.

Old tank syndrome is caused by a loss of buffering capacity resulting in a pH change. The loss of buffering capacity is caused by improper water changes, usually from adding water to the aquarium to replace evaporation loss but not actually removing old water from the tank. Removing old water from an aquarium is a very important part of the water change process. Failure to remove some of the old water at each water change allows organic acids to accumulate. These are produced by the fish and bacteria in the system and are normal. However, when the old water is not removed, the acids use up the buffering capacity of the water (measured by total alkalinity). When the total alkalinity falls to zero, the pH plummets, killing the biofilter and causing old tank syndrome.

Typically, a tank with old tank syndrome will have high ammonia levels (often higher than 10 mg/L), little or no alkalinity, low pH (below 6.0), and high total hardness (several hundred milligrams of mineral per liter). In this situation fish should be moved to a separate holding tank, the aquarium should be totally broken down, everything should be thoroughly cleaned (including the biofilter), and then the entire system should be set up again as a new system. Because it is a new system without an established biofilter, you will need to monitor ammonia levels and change the water as ammonia accumulates until the new biofilter is established.

Anemia is a condition in which the number of red cells in the blood is low. The most obvious sign that a fish is anemic is very pale gills. Observant fish owners may notice this. Although not a common finding, many things may cause anemia. These include various infections and folic acid deficiency, which has been reported in channel catfish. Long-term exposure to nitrite in the water may also lead to anemia. You should consult your veterinarian or other fish health professional if you suspect anemia is a problem in your fish.

Leeches are parasitic bloodsuckers that may carry various blood parasites. Because they consume blood themselves, a heavy infestation with leeches can cause anemia. This does not necessarily mean that blood parasites are also present. Aquariums and ponds usually become infested with leeches by introduction of an infested fish or plant. There are some approved treatments, but avoiding leeches is best, and depopulating infested aquarium or pond fish and restarting the system is very effective.

Digestive Disorders

Parasites cause many digestive disorders in fish. Many parasites live harmoniously in, on, or around fish, but others cause problems such as weight loss and loss of appetite and can lead to death in fry and other young fish. Spironucleus and Hexamita are protozoan parasites that attack the intestines of cichlids, bettas, gouramis, and other aquarium fish. The seriousness of the infection is directly related to the number of parasites present. Crowded conditions, shipping, handling, and other stressful situations can trigger outbreaks. Broodstock and ornamental cichlids should be watched especially carefully. Signs of infection may include lethargy, weight loss, and white, stringy feces.

Another protozoan, Cryptobia, attacks the stomach of African cichlids. Affected fish stop eating and lose weight. There is no treatment for this parasitic infection. Infection of the intestinal tract with protozoa of Goussia species causes lethargy, pale feces, and a high mortality rate in comet goldfish.

Both larval and adult tapeworms are sometimes found in fish. Aquarium fish and carp may be purchased with heavy infections, but they have limited exposure once in the aquarium unless they are fed infected intermediate hosts. Heavily infested fish may lose weight and appear lethargic. Treatment with an appropriate antiparasitic drug may be effective.

Eye Disorders

Eye disease in fish is common and can be caused by several disorders. Diseased eyes may appear swollen, enlarged (as in a pop-eyed appearance), bloody, ulcerated, or otherwise disfigured. A fish’s eyes can be examined with a penlight or bright flashlight to determine if the abnormality is within the eye or in the surrounding tissue. Blood in the eye itself may be caused by injury or infection. Eye injuries commonly occur during transport and handling, especially when fish struggle in a net.

Parasites in the eye, such as eye flukes, may be found in wild-caught fish. Parasitized eyes may appear enlarged and possibly cloudy; sometimes tiny worms can be seen within the eye itself. Infection may compromise the fish’s vision. No drugs have been proven safe and effective for treating eye flukes in pet fish.

Tiny gas bubbles in the cornea (the thin, clear tissue covering the eye) may indicate gas bubble disease. Other signs of gas bubble disease include tiny bubbles in the fins or gills. A gill biopsy can be taken to confirm the diagnosis.

Cataracts (opacity of the lens of the eye) are also common in fish. These can be caused by eye flukes, nutritional deficiencies, or unknown factors.

Bone and Muscle Disorders

Bone and muscle disorders can be caused by nutritional imbalances, including deficiencies in ascorbic acid (vitamin C), vitamin E, and selenium. “Broken-back disease,” indicated by a bent backbone, is typical of vitamin C deficiency, although other problems could also cause deformation of the backbone.

Pleistophora hyphessobryconis is a parasite that attacks the skeletal muscle of neon tetra, angelfish, and other freshwater aquarium fish. Muscle damage leads to abnormal movement. Examination of diseased tissue with a microscope is necessary to confirm the infection. There is no treatment. All infected fish should be removed from the tank to prevent spread of the disease.

Brain, Spinal Cord, and Nerve Disorders

Neurologic disorders can be caused by nutritional imbalances, including deficiencies in thiamine, niacin, and pyridoxine.

Streptococcus infection can cause neurologic signs if it enters the brain. This infection is rare but has been found in rainbow sharks, rosy barbs, danios, and some tetras and cichlids. All fish are considered susceptible. Signs of neurologic disease caused by Streptococcus infection include spinning or spiraling in the water. Sources of infection can be environmental or from infected live foods. The source must be identified and removed to prevent future outbreaks. Antibiotics are usually used to treat Streptococcus infections. Because Streptococcus is in a special group of bacteria, the gram-positive bacteria, specific antibiotics are necessary to treat these infections. Laboratory tests and assistance from your veterinarian or fish health professional are necessary to effectively treat a Streptococcus problem.

Infection with the single-celled fungus Pseudoloma neurophilia affects the nervous system, skeletal muscle, and other organs in zebrafish. Affected fish may have a curved spine, a swollen body, or no signs at all. Fish may be infected from spores in tank debris.

Neurologic disorders may also be caused by ammonia toxicity. This is common in fish with new tank syndrome Heart and Blood Vessel Disorders . Ammonia and pH should always be tested when neurologic disorders are observed.

Nutritional Disorders

One of the most common contributors to illness or death in aquarium and pond fish is improper nutrition. Nutritional disorders occur despite complete, nutritious food being commercially available in a variety of forms, such as pellets, flakes, and granules. One reason for this is that different fish species have different nutritional requirements. Some fish are herbivores (plant eaters), others are carnivores (meat eaters), and many are omnivores (plant and meat eaters). It is very important to learn what diet best matches the needs of your fish. You will probably need to feed more than one kind of food if you have several types of fish in your environment. Improper storage of fish foods is a common cause of nutritional imbalance. Dry food should be kept in a cool, dry place and used or replaced every 2 months.

Many different types of nutritional deficiencies can occur, depending on the nutrient that is missing. Most of these are not diagnosed until the fish are very sick or dead. It is much easier to take the time to learn about and provide the correct diet for your fish than to correct the nutritional deficiency once it occurs.

Feeding live foods to your fish should be done carefully. Wild-harvested live foods may harbor parasites or other harmful organisms that can cause disease in fish. You should purchase any live foods from a reputable source.

Feed intoxications can happen in aquarium settings. The most common of these is a result of feeding fish foods contaminated with the aflatoxin produced by Aspergillus flavus, a mold that grows on the feed. When this aflatoxin is eaten, it causes rapid growth of tumors and a high death rate. Moldy feeds should always be discarded.

Gill Disorders

The special organs called gills allow fish to breathe underwater. Gill disorders can be caused by environmental problems or infections.

Disorders Caused by Environmental Problems

Gas bubble disease can develop when water is supersaturated with dissolved gases. This is common in cold-water systems when cold water introduced into the system is heated too rapidly and the gas in the water builds up in the system. When the fish breathe through their gills, they take in this excess gas. Gas bubble disease has been linked to faulty pumps and, rarely, to ponds with heavy algal blooms that cause afternoon gas levels to be very high. Treatment involves vigorous aeration to blow the excess gas out of the water. Gas bubble disease is easily identified by a veterinarian by observing gas bubbles in gill capillaries when looking at a biopsy sample under a microscope ( See also Heart and Blood Vessel Disorders Heart and Blood Vessel Disorders Many of the disorders and diseases that are known to occur in fish are the result of stress, poor water quality, overcrowding, and failure to quarantine any new or sick fish to avoid spread... read more ).

Carbon dioxide can be toxic to fish when the concentration is greater than 12 mg/L. Fish exposed to high levels of carbon dioxide appear lethargic and nonresponsive. Water is often found to be acidic on pH testing. Treatment is vigorous aeration that blows the excess carbon dioxide out of the water and into the atmosphere. As the carbon dioxide concentration in the water decreases, the pH of the water increases.

Hydrogen sulfide (H2S) is highly poisonous to fish. Excess hydrogen sulfide is usually found in water from a deep well or is the result of accumulated organic debris in the pond or holding tank. Hydrogen sulfide tests are available, but a strong sulfur odor is readily noticeable. Prevention involves thoroughly aerating any water introduced into the system and keeping the system sanitary to minimize buildup of organic wastes.

Disorders Caused by Infections

Dactylogyrus is a common parasite that infests the gills of fish and looks like a small worm under a microscope. It is commonly found on the gills of goldfish, koi, and discus but can also affect many other species of fish. Infested fish brush up against objects to try to remove the worms. Fish become pale, and their breathing becomes rapid and shallow. Gills are swollen and pale. A similar but larger parasite, Neobenedinia, infests salt water fish and can be devastating. These parasites lay eggs and this can cause reoccurring infection even after the adults have been killed. Formalin or praziquantel are most often used to treat these infestations. Quarantine can effectively prevent introducing these parasites into a healthy environment.

Many of the protozoan parasites mentioned under the section on skin disorders can also affect the gills of fish. Microscopic examination of infected tissue is necessary to identify all of these, but treatment for these external gill and skin infections is very similar. Formalin is commonly used. Formalin is toxic; it should be used with caution and by following the instructions for proper use. Other products may be used in some circumstances. Consultation with your veterinarian or fish health professional is recommended.

Bacterial gill disease is seen occasionally in aquarium fish. It usually begins in a system that is overcrowded and has poor water quality. Different types of bacteria are thought to cause this disease. Signs include swollen, mottled, and deformed gills. Because the gills cannot function properly, the fish have respiratory problems. Sanitation is critical to stop this disease and to prevent it in the future. Antibiotics can help treat bacterial infections, but if the underlying sanitation problem is not corrected, the infection is likely to reoccur.

Branchiomycosis is a fungal infection of the gills caused by Branchiomyces species. These organisms are commonly found in decaying organic material in the environment. Branchiomycosis causes respiratory problems and the death of gill tissue. It is typically found in warm ponds and can be prevented by avoiding overstocking and poor sanitation. This infection is extremely rare and has been reported in the United States only a few times.

A very serious herpesvirus infection has been described in koi. The disease is widespread in the United States. The death rate can be close to 100%. The main sign is destruction of the gills, which causes respiratory problems and death. Gills appear white and mottled and are obviously diseased. Laboratory tests to confirm the diagnosis are recommended. Your veterinarian may recommend removal of diseased fish and any fish in contact with diseased fish. Surviving fish can be carriers even if they have no signs and have a negative laboratory test for herpesvirus. If depopulation is chosen, the system should be thoroughly disinfected before restocking. Quarantine of new fish is the best way to prevent introducing koi herpesvirus into an existing fish population.

Skin Disorders

Skin disorders in fish are especially harmful. Any surface injury to the skin makes osmoregulation (fluid balance) more difficult and can lead to circulatory malfunction. The skin and mucus are extremely important protective barriers for fish. They seal the fish so fluid balance is more easily controlled. The mucus allows fish to slip through water more easily, so less energy is used while swimming. There are also several protective compounds in the mucus that protect the fish from bacteria and other organisms in the water. Various types of parasites, from tiny single-celled protozoa to larger lice and worms, can cause skin disorders in fish, as can bacteria, viruses, and other organisms.

Sunburn can be a problem in fish that swim near the surface. Access to shade should be available to fish housed in outdoor ponds. Plants can be an excellent source of protection from direct sunlight.

Disorders Caused by Bacteria

Flavobacterium columnare are bacteria that cause columnaris disease (sometimes called saddleback or cottonmouth disease). Signs may include skin lesions with slimy or cotton-like excretions. It is common in warm-water fish. Early treatment with potassium permanganate can help, but if the disease is not recognized in the early stages, antibiotic treatment is generally needed.

Disorders Caused by Viruses

Carp pox is one of the oldest recognized fish diseases. It is caused by cyprinid herpesvirus-1. It is primarily a disease of koi. The skin abnormalities are smooth and raised, possibly with a milky appearance. They do not usually cause problems, but they can be a site of secondary bacterial infection. Carp pox is of particular interest to koi enthusiasts because the high value of these fish is based on appearance. Quarantine is essential, and any infected koi should be removed. Surgery to remove the pox lesions is not helpful.

Lymphocystis disease is a viral infection that can affect both saltwater and freshwater fish. In home aquariums, painted glass fish are especially susceptible. The main sign is cauliflower-like growths on the skin or fins. Microscopic examination of tissue is necessary to confirm that the problem is lymphocystis. These growths do not usually cause a health problem but do affect the appearance of the fish. Infections usually are not life-threatening and resolve without treatment.

Disorders Caused by Protozoa

A common protozoal infestation in home aquariums and ponds is ich, or white spot disease. This disease is caused by Ichthyophthiriusmultifiliis in freshwater fish and by Cryptocaryon irritans in saltwater fish. The organisms attach to the fish and burrow into the skin and gills. The resulting cysts appear as visible white spots. Microscopic examination of diseased tissue is required to confirm the diagnosis. As the protozoa begin to reproduce, they create a wound in the fish and sink to the bottom of the tank or pond to reproduce. One parasite can produce hundreds of new organisms. This rapid and massive reproduction is what makes this parasite so deadly. The damage caused by large numbers of these parasites can make a fish more susceptible to infection by other agents or cause a loss of bodily fluid. Treatment is with either copper sulfate or formalin, both of which can be obtained from your veterinarian or pet store. As with all medications, label instructions should be followed carefully. The treatment must be applied at specific intervals based on the water temperature, so advice from a fish health professional is recommended.

Another serious disease in aquarium fish is called velvet (also known as rust or gold dust disease). It is caused by protozoan parasites that attack the gills and skin, causing fine yellowish spots that are smaller and harder to see than the ones that occur with ich. Sometimes these appear as a thin, velvety film covering the skin. Other signs include loss of appetite, lethargy, and a tendency for affected fish to scratch against rocks or other hard objects. High death rates are common. Both freshwater and saltwater fish may be affected. In freshwater fish, the cause is Piscinoodinium (also known as Oodinium). In saltwater fish, a related parasite called Amyloodinium is responsible. The parasites attach to the skin and gills. Piscinoodinium and Amyloodinium can be identified by microscopic examination of gill, skin, or fin tissue. Your veterinarian may recommend chloroquine, an effective treatment for ornamental fish (chloroquine cannot be used for food fish). Instructions for giving it must be carefully followed. A recheck of the fish in 7 to 10 days may be needed.

Skin diseases, fish

Skin diseases, fish

Other protozoan parasites such as Chilodonella, Brooklynella, and the trichodinids may infest the gills or skin of aquarium fish. Infected fish may seem to have excessive amounts of slime or mucus. Microscopic examination of diseased tissue is required to confirm the diagnosis. Signs include dulled coloration, a light gray-white covering of mucus on the body of the fish, gill damage, and general weakness. Fish often rub against objects to relieve itching. Other observations may include rapid breathing, piping (swimming near the surface of the water, trying to gulp air), flashing (scratching), and loss of condition. Once the diagnosis is confirmed, fish can be treated. Formalin is often effective. The trichodinids in particular are often associated with overcrowding or poor sanitation, in which case cleaning the system is an important component of the treatment plan.

Tetrahymena corlissi and Uronemaspecies are parasites that are usually found on the skin, gills, or fins, but they can also be found inside fish, including in skeletal muscle and the fluids of the eye. These parasites are usually found in water that has a high level of organic matter. If the parasites are only on the surface of the fish, the infestation can be cleared with good sanitation and chemicals. If the parasites have moved inside the fish, the condition is not treatable and is often fatal.

Ambiphyra and Apiosoma are parasites that attack the skin and gills. These parasites are more common in pond fish than in aquarium fish and usually do not affect saltwater species. Low numbers of these parasites are not a problem, but high numbers can damage the skin and gills, which compromises breathing and can leave the fish susceptible to other infections. Treating involves using formalin, copper sulfate, potassium permanganate, or salt. Excessive crowding and poor sanitation are predisposing factors and should be avoided.

Ichthyobodospecies are common parasites of the skin and gills of aquarium, pond, and saltwater fish. They can be difficult to see but look like a flickering flame when infected tissue is examined with a microscope. Affected skin may appear to have a steel-gray discoloration, and mucus production called “blue slime” or “gray slime” may be seen. Behavioral signs include lethargy, poor appetite, piping (swimming near the surface, trying to gulp air), flashing (scratching), and overall weakness and loss of condition. Microscopic examination of infected tissue is required to confirm the diagnosis. Poor sanitation, crowding, or overfeeding can contribute to proliferation of the parasites and should be corrected. Salt, formalin, copper sulfate, or potassium permanganate baths can be effective treatments after underlying problems have been corrected.

Disorders Caused by Larger Parasites

The anchor worm (Lernaea) is a parasite that buries its head into the muscle tissue of a host fish. Despite the name, anchor worms are not worms, but crustaceans. These parasites can damage gill tissue. Pond fish are the most common hosts. The fish may scratch against objects to try to knock the parasite off. The parasite can easily be seen, appearing as whitish-green threads hanging off the red, inflamed skin of the fish. The parasite can be removed manually. Treatment is recommended to eliminate the parasite from the pond or aquarium.

Fish lice (Argulus) and leeches are parasites that attach themselves to a host fish, penetrate the skin, and feed on the blood. The fish scratch against objects to try to remove the lice. Lice look like small, clear disks that lie flat against the skin. Leeches are worm-like parasites that contract when touched. Treatment involves removing the parasites from the fish. The tank should be treated to kill any lice larvae that may be present. Leech infestations can be seasonal, but eggs may have been laid in the system, resulting in reinfection after the adult parasites have been eliminated. Treatment is usually recommended if the system has no contact with surface water.

Gyrodactylus and Dactylogyrus are tiny flatworms that are skin and gill parasites of goldfish, koi, and other fish. The parasites are usually too small to be seen without a microscope. Fish become pale and can have skin sores with scattered hemorrhages and ulcerations. The death rate can be high when fish are heavily infested. Formalin or praziquantel are the treatments most often used for these infestations, and quarantine is a good practice to help prevent introduction of these parasites into a healthy environment.

Some saltwater parasites (Neobenedinia and related capsalid parasites) that have a similar effect as Gyrodactylus and Dactylogyrus do in freshwater fish. The marine organisms can be more virulent, however, because they are much larger and therefore can cause more damage to the fish. They also lay very sticky eggs that can be easily spread by nets or other objects to uninfected systems. Praziquantel is an effective treatment for adult ornamental fish (not for food fish). Reinfection remains a threat if eggs have been released into the system.

Kidney and Urinary Tract Disorders

Sphaerospora auratus is a parasite that causes renal dropsy. It attacks the kidneys of pond-raised goldfish and can lead to severely enlarged, cystic kidneys and death. There is no practical treatment. Fortunately, infection is extremely rare.

Disorders Affecting Multiple Body Systems

Some disorders are widespread, affecting multiple organs and body systems. The most common types found in fish are discussed below.

Disorders Caused by Bacteria

Aeromonasinfection is the most common bacterial infection of freshwater aquarium fish. Fish infected with Aeromonas or other closely related bacteria may show signs that include bloody spots or ulcers on the body, fluid accumulation in the abdomen (“dropsy” and “pinecone disease”), ragged fins, or enlarged eyes. Diagnosis of a bacterial infection requires laboratory testing. Many bacterial infections can be treated with antibiotics, but laboratory testing is necessary to determine which antibiotics will work against the particular bacteria causing the problem. Koi and goldfish are especially susceptible to a certain type of Aeromonas called Aeromonas salmonicida. This particular bacteria can cause deep ulcers and death in these fish. Antibiotic testing in a laboratory may be the only way to determine which drug should be used to control an outbreak of this disease.

Vibriosis is a common and potentially serious disease that affects many types of fish, although it is less common in freshwater species. Vibrio bacteria are responsible for causing bleeding and ulceration of the skin, fins, and tail. Internal organs can also bleed and break down. Preventive steps include minimizing stress and crowding. Antibiotics are useful but laboratory tests are needed to determine which ones will be effective. Bacterial populations change, and an antibiotic that worked in a previous outbreak may or may not be effective in a later outbreak. Rapidly changing antibiotics or “shotgunning” fish with many different drugs is a dangerous practice and can result in creation of resistant bacterial strains.

Edwardsiellosis, caused by Edwardsiella tardaor other Edwardsiella species, causes intestinal disease and skin ulcerations in many species of fish and can also infect reptiles, amphibians, birds, and mammals (including people). Signs include gas-filled lesions that have a bad odor when they burst. Affected fish cannot swim normally because the gas-filled lesions affect their floating ability. Antibiotics are effective, but laboratory tests are needed to determine which drug will work best.

Mycobacteriosis is a bacterial infection that can affect aquarium fish. These bacteria are generally resistant to antibiotic treatment in fish. Fish may be infected by eating infected dead fish or by other routes of contact. Periods of stress and high numbers of mycobacteria in the water predispose fish to infection. These organisms are common in our environment. Poor sanitation, low oxygen, and low pH create conditions that are favorable to these bacteria, increasing the threat to fish in such an aquarium. Diagnosis of Mycobacterium is difficult because signs resemble those of many other diseases. Mycobacteria form granulomas (nodules) within the tissues. Signs could include:

  • weight loss

  • skin ulceration and bleeding

  • paleness

  • skeletal deformity

Your veterinarian can diagnose Mycobacterium infection by examining different tissues with a microscope. Tissue may also be submitted for special laboratory tests to confirm the diagnosis. Infected fish are usually destroyed. The environment needs to be thoroughly cleaned with alcohol or phenolic compounds before starting over. Mycobacterium can infect people, causing skin infection that can progress to more severe illness. People with immune system disorders should not be allowed contact with a Mycobacterium-positive aquarium. It is always wise to wash your hands after working in any aquarium, and people with compromised immune systems should probably not handle aquariums directly.

Disorders Caused by Fungi

Saprolegnia can affect fish and fish eggs. Signs include grayish-white, cotton-like growths on the skin, gills, eyes, or fins. This fungus can spread to the internal organs and deeper tissues of the body. Prevention consists of removing all potential sources of the fungus, such as correcting poor sanitation and removing any dead and decaying matter. If the environment is clean and skin pathogens have been eliminated, a single treatment of potassium permanganate can often control external Saprolegnia.

Ichthyophonus hoferi is a fungus that causes internal infections, usually in older aquarium fish. It is usually identified through examination of fish that have died. Prevention includes removing infected fish and avoiding feeding raw fish products. This infection is very rare.

Cancer and Tumors

Like other animals, fish can develop cancer. The incidence can be higher in certain geographic areas and in certain species. Some tumors, such as the malignant melanoma of the gypsy-swordtail cross and fibromas or sarcomas of goldfish, are genetically linked and controlled. Tumors of the reproductive organs are common in koi. Affected fish have a swollen abdomen and lose body condition. If the fish is not severely debilitated, some tumors can be removed surgically. Some viruses also cause cancer in fish.

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