Under ideal circumstances — proper water conditions, a diet including a variety of foods, uncrowded conditions, and an environment lacking other stress — diseases rarely affect fish. Usually fish will only get sick when something in the environment is not right, or when new fish is added to the tank. Healthy fish generally have strong immune systems and are capable of resisting most pathogens, but when stress weakens the fish, the fish becomes more susceptible to illness. There are several circumstances, including non-living and living factors, that can lead to fish stress.


Among non-living factors that can adverse affects on aquarium (and terrestrial) inhabitants include poisoning, incorrect temperature or pH, and a shortage of oxygen.

    Ammonia Poisoning
    Ammonia poisoning is caused by the buildup of organic waste due to overfeeding, fish or plant deaths and decay, or improper cycling. Ammonia poisoning especially occurs when the pH exceeds 7, when benign ammonium becomes ammonia. Symptoms of ammonia poisoning include sluggish behavior, panting, and gill discoloration (gill burn). Fish may hang just below the water surface. The easiest way to confirm ammonia poisoning is by testing the water. Ammonia poisoning can be reduced by reducing feedings, making water changes, lowering the pH, using zeolites, and increasing aeration.

    Nitrite/Nitrate Poisoning
    Nitrite/Nitrate poisoning is caused by the same activities as ammonia poisoning. Nitrite/Nitrate poisoning has the same symptoms as ammonia poisoning, and can be tested by a Nitrite/Nitrate water test kit. The best course of action, is to reduce feeding, make frequent partial water changes until the compounds are reduced, and increase the aeration in the water.

    Chlorine Poisoning
    Free chlorine, present in most tap water, is toxic to fish. Chlorine affects the gills and causes death by asphyxiation. Chlorine can be removed by boiling the water, letting the water stand for a few days, vigorously aerating the water, or by adding a water conditioner.

    Heavy Metal Poisoning
    Heavy metal poisoning can result from old pipes and/or metal in the fish tank. Heavy metal poisoning is evident when fish gasp at the surface for air and breath rapidly. Tests are available to measure the amounts of heavy metals in your water. The best way to remove heavy metals is to utilize a reverse osmosis system, although filtering the water through activated carbon and using water conditioners can be substituted.

    Hydrogen Sulfide Poisoning
    This gas is caused by rotting debris and waste in the gravel of the tank. This gas is toxic and the first signs of its presence is a rotten egg-like odor and fish gasping at the water surface for air. The best measure to take is to make a large water change, using a siphon to remove waste from the gravel. Make partial water changes until the odor is gone and the fish return to normal swimming and breathing.

    Medication Poisoning
    Medications are meant to help fish recover, although when misused, can be harmful than helpful. Medications can have adverse affects on many types of fish including catfish, tetras, Mormyrids, Loaches, and other sensitive fish. Copper-based medications have harmful affects on invertebrates, so always remove snails and crustaceans from the tank before treating it. Always be sure to read the label on the medication to confirm that it is suitable for your fish. If a medication appears to be harming your fish, make a partial water change and filter the water with activated carbon.

    Other Pollutant poising
    There are other chemicals (cigarette smoke, paint fumes, pesticides) that sometimes make their way into the fish tank. The best way to combat these pollutants is not to allow them to get in the tank in the first place. However, once a foreign pollutant enters the tank, the results can be drastic for the inhabitants. Try making water changes and filtering with activated carbon to alleviate the problem.

    Incorrect Temperature
    Most tropical fish are used to living in water with a small temperature variance. When the temperature drops below or exceeds this range, fish can be weakened and left more vulnerable to disease. The best way to prevent wide variances in temperature is to purchase a reliable heater and place the tank away from drafty areas.

    Incorrect pH
    Most tropical fish live in water with a relatively stable pH. When the pH is not right, the fish are weakened and become more susceptible to illness and infection. Symptoms of an improper pH include darting movements, inflamed and bleeding gills, rapid gill movements, and fish hanging just below the water surface gasping for air. If the pH is way off, do not rapidly restore the pH to normal. Instead, gradually add pH buffers until the proper pH is reached. To help prevent this problem, check the pH on a regular basis.

    Oxygen Shortage
    A shortage of oxygen can be diagnosed by rapid gill movement and fish hanging just below the water surface. Later, the fish may lose color and die. An oxygen shortage can be caused by several ways: insufficient aeration, a buildup of organic wastes, a high temperature, or through plant respiration. An oxygen deficiency can be solved by a partial water change, an increase in aeration, and removal of dead or dying fish and vegetation.

Sources of stress to fish include living pathogens such as parasites, bacteria, fungus, and viruses (least common). These pathogens are most likely to affect fish when other conditions are weakening the fish, or when a new, infected fish is introduced. Other fish also serve as sources of stress when they attack other fish. "S" = symptoms; "A" = action.

    - - - - - - - -

    Velvet Disease (Oodinium)
    S: Velvet disease is characterized by a cover of a fine gold to gray film on the fish's body. The affected fish may gasp for air and rub against rocks. The gills may be flared.
    A: When Velvet disease has been confirmed, raise the water temperature of the tank. If possible, the temperature should be brought to 88-93�F (31-34�C). Turn off the lights and treat fish with copper sulfate, Trypaflavine, methylene blue, a malachite green-formalin combination, or Quinine Hydrochloride. An alternative is to immerse the infected fish in a salt bath.

    White Spot Disease, Ich (Ichthyophthirius)
    S: The body and fins are covered with small white spots. In heavy infestations, the skin may be covered with slimy gray patches. As the disease progresses, the fish becomes emaciated and less active, and scratches against objects.
    A: The Ichthyophthirius parasite has three life cycle stages, and is only vulnerable to medication in the free-swimming stage. Raise the temperature to 86�F (30�C). This highly contagious disease is treatable with a malachite green-formalin combination, Trypaflavine, Quinine, an addition of salt, or one of many medications available in pet stores. Treat the tank for three weeks so all the Ich parasites have completed their cycles. Survivors appear to be immune after infection, but still carry disease. Non- immune tank mates are infected when they are weakened by stress.

    Gill Flukes (Dactylogyrus, Cichlidogyrus, Tetraonchus)
    S: Tiny worm-like flukes infest the gill membranes causing redness and slimy gills, panting at the surface, rapid breathing, and emaciation.
    A: Flukes lay eggs that are resistant to medication. Treat the tank with Droncit (Praziquantel) at 2 ppm (2 Mg/L) or a malachite green-formalin combination until all eggs and flukes are gone. Flukes can be eliminated from fish (but not the tank), by short formalin, salt (3 %), or Ammonium Hydroxide baths.

    Skin Flukes (Gyrodactylus)
    S: Fish are infested with small worms causing colors to fade, reddish patches, and the fish to scratch against objects.
    A: Skin flukes can be treated with Droncit (Praziquantel) at 2 ppm (2 Mg/L) and formalin baths.

    Fish Lice (Argulus)
    S: Fish rub against rocks and plants and have clamped fins. Areas may have red, inflamed spots. Small lice are visible to the naked eye.
    A: The best treatment is careful, manual removal with a pair of tweezers and siphon the gravel daily to remove eggs. If there is a heavy infestation, raise the temperature to 86�F (30�C), and treat the aquarium with dylox, masoten, or trichlorfon (0,0-dimethyl- 2,2,2-trichloro-1-hydroxyethyl phosphonate). Be sure to remove all invertebrates from the aquarium as they may be harmfully affected by the treatment.

    S: The body is covered by a gray layer of slime and the fins are frayed. The fish may swim erratically and rub against rocks. Heavy infestations result in reddish patches on the skin. A highly infectious disease that thrives in acidic water.
    A: The temperature should be raised to 86�F (30�C) and the fish should be bathed in a short formalin, or a longer salt bath. Medications are available in pet stores.

    S: The body and gills lose color and become gray-blue in color. Fish swim erratically and rub against objects.
    A: A malachite green-formalin combination works well as does a short (30 min) salt bath followed by a 12 hour Trypaflavine bath. This disease is most prevalent in overcrowded tanks and is most easily prevented by maintaining an appropriately stocked tank.

    Hole-in-the-Head Disease, Discus Disease (Hexamita, Spironucleus)
    S: Diseased fish lose weight and develop "pitting" in the head region.
    A: The cause of this disease is usually attributed to a lack vitamins and minerals, especially vitamin D and calcium, the effects of the deficiency possibly amplified by the presence of Hexamita in the intestines. This intestinal flagellate is usually introduced with the feeding of Tubifex worms, and can survive in the gravel of unclean tanks. The best way to cure Hole-in-the-Head Disease is to complement the diet with vitamins. Treatment can also include dosages of antibiotics and metronidazole. Keeping the tank scrupulously clean also helps prevent this disease. Try medicated fish foods.

    - - - - - - - - - - - -

    Saprolegnia and Achlya
    S: Cotton-like tufts of fungus appear on the body of the fish. As algae grows on the fungus, the fungus turns a brownish color.
    A: Fungal infections are secondary infections that can only occur when the fish is already diseased or physically injured. When fungal infestations occur, raise the water temperature. Several different courses of action can be taken including a 30 minute bath in 1 ppm Sodium Permanganate (1 Mg/L); the addition of Malachite green; or the addition of a commercial fungal medication.

    - - - - - - - - - - - - - - -

    Fish Tuberculosis (Mycobacterium)
    S: Fish may lose color and appetite and become hollow-bellied. Fish become lifeless and often crippled—with a bent spine. Fish develop ulcers under the skin and may rupture causing open sores and "pop-eye."
    A: This disease is highly infectious and deleterious. Bacteria can remain living in the gravel to infect other fish when they are weakened. Some success has been achieved by treating infected fish with antibiotics (Oxytetracycline and Kanamycin), although often it is best to kill the fish and put it out of its misery. When removing piscine tuberculosis victims, do so with care, as this disease can be transmitted to humans.

    Dropsy (Aeromonas, Pseudomonas)
    S: Fish infected with Dropsy are characterized by protruding scales, bulging eyes, pale gills, the body cavity is filled with fluid, and there may be red patches on the skin.
    A: An infectious disease that spreads most readily among weakened fish. Infected fish should be removed and destroyed, or treated with antibiotics. Treatment is not usually successful.

    Fin Rot (Pseudomonas and others)
    S: The edges of the fins are discolored and frayed. As the disease progresses, fin damage becomes more evident as the fins disintegrate. Often fungal infections follow fin rot, contributing to fin damage.
    A: Fin rot is most commonly caused by improper water conditions including too low a temperature and the buildup of toxic compounds. This infectious disease can be treated with a bath in Trypaflavine or the addition of commercially prepared medications. Treatment of Fin rot is difficult.

    Mouth Fungus, Columnaris (Chondrococcus, Cytophaga)
    S: Despite its misleading common name, mouth fungus is actually caused by bacteria. Patches of cotton-like patches develop around the mouth, but also on the head, fins, gills, and body. As the disease progresses, open sores develop.
    A: Raise the water temperature. Several different courses of action can be taken including a 30 minute bath in 1 ppm potassium permanganate (1 Mg/L); the addition of Malachite green; or the addition Nifurpirinol. Frequent partial water changes are important.

    Neon Disease (Sporozoasis)
    S: This incurable disease can affect characins, Cyprinds, and cichlids. The disease can manifest itself in several ways. Symptoms vary, and can include a loss of color, emaciation, and the loss of equilibrium causing fish to swim in an erratic, jerky manner. An infected specimen will wander from its school. The body may become a milky, opaque color.
    A: Since this disease is not treatable, the best way to prevent its spread is to immediately remove the affected fish, and destroy it. Disinfect the tank after removing other fish.

    Discus Flu, Discus Plague
    S: The disease sets in rapidly after new fish are introduced. The first symptoms include small white patches on the body and the disintegration of the fins. Soon, the milky mucous membrane begins to slough off in large sections and the fish turn a dark color. If a number of fish are affected, they may huddle together in the corner of the tank.
    A: This seasonal, flu-like disease, that has devastated entire hatcheries, is caused by seemingly unknown causes. Scientists have found a number of species of bacteria in diseased fish. The best way to combat the disease is to keep the pH low (4-5), make daily water changes (clean the tank well), and stop feeding the fish. The lights should be turned off and decorations removed. Try treating the water with a small amount of potassium permanganate to kill off some bacteria. Some suggest using an antihistamine throughout the course of the disease.