Bacterial Infections

The most common bacterial infections are caused by one of three pathogens: Vibrio, Pseudomonas or Aeromonas. The symptoms (e.g., cloudy eyes, bloody patches, decaying or frayed fins, scratching) of these bacterial infections can be similar, and therefore it can be difficult to determine which pathogen is responsible. Fish with an internal bacterial infection may not show any signs other than a loss of appetite and possibly a swollen abdomen.

With most bacterial infections, all fish in the aquarium will be affected to varying degrees, so the entire aquarium may need to be treated. Obviously, if we are dealing with a large aquarium, this could potentially be expensive. If only one fish appears to be infected and you move the infected fish to a quarantine aquarium as soon as possible, you may get lucky and not have to treat the display aquarium. 


Dropsy is not a specific disease, but rather a symptom of a deteriorated health condition. With dropsy, the fish will have visible swelling and projected scales. This is the result of a fish not being able to regulate the amount of fluid in a part of its body. The affected area is typically the abdomen; specifically, it is most often the visceral cavity that houses a number of organs, such as the stomach, intestines, gall bladder and kidneys. The failure to regulate fluids is a symptom; therefore, there is usually some other disease involved that starts the process (caused by poor water quality, stress, internal bacterial infections, parasites, viruses and tumors). Although dropsy is fairly easy to diagnose, the cause is much harder to determine; however, the primary cause is usually attributed to a bacterial infection. The causative agent can be introduced to the aquarium through food, poor water quality or through the introduction of other fish to an established aquarium. Although dropsy is not highly contagious, the affected fish should be removed and placed in a quarantine aquarium. Dropsy can be spread from the affected fish, which can possibly produce stress among the other fish and make them more vulnerable to dropsy or other conditions.

Unfortunately, fish affected with dropsy is not very good. By the time the fish has swollen up and the scales project outward, the internal damage may be too extensive to repair and for the fish to recover. Most cases of dropsy are fatal.


If a fish appears lethargic and exhibits a loss of equilibrium, it may have a fungal disease: Ichthyophonus. Although Ichthyophonus fungi are generally considered a fungal disease of marine fish, it does show up in freshwater fish from time to time. Infected fish become lethargic, and if the brain is infected, they may exhibit a loss of equilibrium as well as staggered movements. 

Freshwater Ich

The most common symptom of freshwater ich (Ichthyophthirius multifiliis) is the presence of small white spots (trophonts) on the body.

The most common symptom of freshwater ich (Ichthyophthirius multifiliis) is the presence of small white spots (trophonts) on the body. Actually, these "white spots" are thickened masses of protective mucus that have covered the attacking protozoan in an attempt to dispel it. Additional symptoms include rapid breathing, cloudy eyes, possible fin deterioration and flashing. The life cycle of ich includes a host organism and the environment. The trophont is the encysted feeding stage of the parasite which enlarges, breaks through the epithelium and eventually settles on the bottom of the aquarium. When on the bottom of the aquarium, the organism, which is now referred to as a tomont, begins to undergo mitosis (cell division) and produces hundreds of ciliated theronts. If the theronts encounter a host fish, they will attach, penetrate and enlarge (and therefore be visible to the aquarist as white spots).

Treating ich is possible with the use of heat. Ich thrives in a temperature range of 70 to 75 degrees Fahrenheit. The trick is to slowly increase the water temperature to approximately 86 to 88 degrees over the course of several days and leave it at this elevated temperature for approximately 10 days. The elevated water temperature is usually enough to kill the heat-sensitive theronts. A temperature above 84 degrees is the upper limit tolerated by the parasite. Owing to lower dissolved oxygen levels at elevated water temperatures, it is imperative that additional air be supplied to the aquarium using several airstones. If you don't want to subject the entire display aquarium to elevated water temperatures or medication that might stress plants, etc., move the fish to a quarantine aquarium for treatment. After all fish (the host animals) are removed from the display aquarium, the theronts will eventually die due to the lack of a host. Meanwhile, the infected fish can be treated using heat or malachite green. Malachite green do not penetrate and kill the trophonts, but instead prevent the motile trophonts from reinfecting the fish. Ultraviolet sterilization can also be used successfully in controlling ich in its free-swimming stage.


If a fish has growths resembling raspberries, it may be infected with Lymphocystis. The tumors are caused by a viral infection and in some cases, a variety of environmental factors, such as poor water quality. Lymphocystis can be inherited by the parent fish or transmitted to other fish through abrasions on the skin. Lymphocystis is rarely fatal. Some hobbyists have had limited success in surgically removing the tumors and swabbing the area with some kind of iodine preparation - but there is no guarantee that they won't grow back.

Because this is a viral infection, there is no real cure, and most people usually isolate the infected fish and let the infection run its course. 

Hole in The Head Disease (HITH)

Hexamita can build up under the skin around the head of infected specimens, which may lead to localized areas of tissue breakdown and possibly hole-in-the-head disease.

Hexamita, which is a small parasite about the same size as a red blood cell, oval in shape, with two nuclei, six flagella on the front of the body and two flagella at the rear end. The parasite initially infects the intestines but can rapidly spread to the liver and the blood. Symptoms of discus infected by Hexamita include pale-colored, muscus like feces, loss of appetite and emaciation. Hexamita can build up under the skin around the head of infected specimens, which may lead to localized areas of tissue breakdown and possibly hole-in-the-head disease. In untreated specimens, entire areas of skin may be undermined, resulting in large open sores. It is still debatable as to whether Hexamita is a primary or secondary cause of hole-in-the-head disease.

One of the simplest treatments for Hexamita is to slowly increase the water temperature to 86 degrees for five days, followed by a water change. The increased water temperature weakens the parasite while strengthening the fish immune system. If only a single specimen is infected, it should be isolated in a quarantine aquarium where the treatment is carried out.

Pop Eye

Pop-eye is characterized by the eye protruding from the socket, and it may appear inflamed. Pop-eye is usually caused by bacterial septicemia, tuberculosis, parasites or as a result of oxygen supersaturation of the water. Oxygen supersaturation occurs whenever the pressure of a gas in the water is higher than the pressure of the same gas in the surrounding atmosphere; the difference in gas pressures causes the gas to get pulled too quickly out of the fish's bloodstream, leaving behind gas bubbles.

Treatment includes improving the water quality, maintaining excellent water quality thereafter and possibly reducing aeration of the water. Unfortunately, if tuberculosis or parasites are involved, the condition is usually incurable, and the individual should be removed and euthanized.


If a fish has a bent or curved spine, it is most likely infected with a Gram-positive mycobacteria (Mycobacterium marinum or M. fortuitum). This is commonly referred to as fish tuberculosis, piscine tuberculosis, acid-fast disease or granuloma disease. Tuberculosis is a chronic, progressive disease that may take years to fully develop. Symptoms include lethargy, emaciation, fin and scale loss, exophthalmia (bulging eyes), skin inflammation and ulceration, edema (dropsy), peritonitis (parasite infestation) and nodules in muscles that may cause deformation of the fish. Fish that appear to be most susceptible to fish tuberculosis are gouramis, black mollies, neons, and other tetras, carp and anabantids.

Infected fish should be removed and quarantined immediately for four weeks or more. To prevent this infection, do not overcrowd, and provide good water quality. Remove any fish that appear affected. 


Velvet disease in freshwater fish is caused by the protozoan Piscinoodinium. The velvet parasite is classified as a parasitic algae because it contains chlorophyll and therefore obtains some of its food through the chlorophyll. For this reason, it is often suggested to darken your tank if your fish exhibit a velvet outbreak, as chlorophyll requires visible light to survive. Velvet-infested fish exhibit small yellowish spots that are much smaller than ich spots. Similar to fish infested with ich, fish with velvet may exhibit clamped fins, and they may flash off of rocks and other surfaces in an attempt to dislodge the parasites. If the gills are affected, the fish may exhibit rapid respiration or gasp for air at the surface.



Disease Prevention

The best defense against disease is prevention. It is in your best interest to adhere to the following disease-prevention criteria.

- Provide the best environment for your fish to avoid stress on them. Educate yourself about any special needs a species may have.

- Maintain excellent water quality and perform frequent water changes.

- Monitor water quality weekly and keep a log to monitor changes.

- Avoid overcrowding.

- Feed a balanced and varied diet consisting of commercially prepared foods (e.g., flake, frozen, freeze dried), supplemented with live foods, and avoid overfeeding.

- Quarantine all new fish for a minimum of four weeks.

- Observe your fish regularly to monitor changes in behavior or for symptoms of disease.



A quarantine aquarium is inexpensive to operate and can be set up easily. For the vast majority of freshwater aquarists, a 15-20 US gallon aquarium with a sponge filter, heater and some sort of flowerpots to provide ample hiding places are sufficient, unless of course you are maintaining some of the larger freshwater species.

While fish are held in quarantine, excellent water quality must be maintained in respect to pH and nitrogenous waste levels. Although the ammonia and nitrite levels should be zero if the sponge filter has a sufficient culture of beneficial bacteria (Nitrosomonas and Nitrobacter), the nitrate level and proper pH can easily be maintained through frequent water changes. When performing these water changes, it is important that the pH and temperature do not fluctuate, in order to avoid further stress.

In general, quarantining fish for 30 to 60 days will prevent introducing most parasites into the aquarium. During this period, water changes alone will assist in eliminating the parasites by dilution and minimizing reinfection.