Common Tropical Fish Diseases

By: Chewy EditorialPublished:

Common Tropical Fish Diseases

You walk by your aquarium, and something catches your eye. You stop and turn to take a closer look – something doesn’t look right. Upon closer examination, you see that a fish is not behaving normally, or there are external signs of disease. If you have been keeping fish for any length of time, you have experienced a similar scenario.

Common Signs of Possible Disease

Altered or Decreased Activity

These are common signs of disease, but they are not specific to one disease and may be seen with poor water quality, various infectious diseases and many other problems.

Clamped Fins

This is a nonspecific sign that can be seen with a variety of diseases. Flashing or glancing. The act of fish rubbing up against objects in the aquarium may be a sign of irritated skin. This is commonly observed with fish afflicted with external parasites.


With fish that normally school, an individual isolating itself from the group can be an early indication of disease.

Respiration Concerns

Gasping for air at the surface is a sign of either poor water quality (including low dissolved oxygen), or gill disease or gill damage (parasites, bacteria, viruses). Also look for increased breathing rates and crowding near the filter return at the top of the water column.

Skin Lesions

Skin lesions are one of the most common signs of disease and are often the first problem observed by hobbyists.

Common Freshwater Fish Diseases

If you observe your tank and see one of the symptoms mentioned earlier, it’s likely that the fish has one of these common freshwater fish diseases. Once you can diagnose the problem (or make an educated guess as to what it could be), you can treat the fish.

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. Over the past 25 years, I have used a variety of medications with varying degrees of success; however, in most cases, tetracycline has been the most effective treatment for infections caused by both Vibrio and Aeromonas, though there are other antibiotics that you can use. Remember to follow the manufacturer’s directions and remove any activated carbon from your filter before treating the aquarium. Another option is the use of medicated foods if the fish are eating well.


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. Although there are no present medications that can effectively cure fish stricken with dropsy, your first line of defense is to administer a wide-spectrum antibiotic in the condition’s early stages. In addition, you might add Epsom salts to your aquarium (20 milligrams per liter or 75.2 milligrams per gallon), which will aid the affected fish in expelling unnecessary fluids from its body.

Unfortunately, the prognosis of 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. As far as I know, there is no treatment for this fungal infection, but fluconazole, which is a relatively new antifungal agent active against Saprolegnia fungi, may also be affective against Ichthyophonus fungi. Fluconazole should be administered at the rate of 6 mg/L (22.6 milligrams per gallon) daily for five consecutive days. If there is no improvement over the course of two weeks, you might consider euthanasia.


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).

In the past, I have been successful in treating ich 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, malachite green, formaldehyde or a number of products available at your local pet store. Malachite green and formaldehyde 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; however, a UV sterilization unit is a more expensive route to take.


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. Another option is to try acriflavine, which will kill plants. Therefore, in addition to following the manufacturer’s instructions, treat your fish in a quarantine aquarium if you have live plants in your display aquarium.

Parasites (internal)

If a fish exhibits a loss of appetite, emaciation, and in severe cases, parasites protruding from the vent, it is infected with parasites. In the past, I have had very good results using medications that include malachite green and formalin. In addition, I suggest metronidazole, quinine sulfate, nitrofuracin green or some other readily available medication for treating parasites.

A common parasite in discus is 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, muscuslike 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 discus’ immune system. If only a single specimen is infected, it should be isolated in a quarantine aquarium where the treatment is carried out. In more severe cases, dimetridazole and metronidazole have both shown to be effective against Hexamita. In the United States, products such as metrozol and other similar medications may be available at your local pet store.


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. If the cause is bacterial, treat the aquarium with a wide-spectrum antibiotic. 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. Some successful treatments have been described using chloramine-B or -T, cyclosporine, doxycycline, ethambutol, ethionamide, isoniazid, kanamycin, minocycline, penicillin, rifampin, streptomycin, sulfonamides and tetracycline. In addition, you might try using streptomycin for the first four days at a dose of 10.6 mg/L (40 mg/gallon). After the streptomycin treatment is completed, feed the affected fish with food that has been treated (soaked in) with rifampin at a rate of 10 milligrams per 100 grams of food for about two months. At the same time, treat the aquarium with isoniazid twice a week at a 10.6 mg/L (40 mg/gallon) dosage for one month. If all fish become infected and eventually die, the entire aquarium should be sterilized using a mild bleach solution and rinsed with liberal amounts of water before adding any new fish.


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.

The velvet parasite has two life stages: a free-swimming form and a cyst form. The infective stage of this parasite is the free-swimming stage. During this stage, the velvet parasite has two flagella that enable it to propel itself through the water. It propels itself through the water until it finds a suitable host. Then it will attach itself to the skin or gills of the fish. Eventually, the parasite will form a cyst on the fish, which will remain on-site until it releases several hundred free-swimming Piscinoodinium. These newly released Piscinoodinium go in search of another host, and the cycle begins again. For this reason, velvet is very contagious.

Fortunately, there are effective treatments for velvet. Copper sulfate appears to be the best treatment. The only downside to using copper sulfate in your aquarium is that it will kill any invertebrates, such as snails and shrimp. In addition, it is very important not to overdose with copper sulfate, as this compound can easily poison and kill fish. Consequently, once the copper-sulfate treatment is completed, gradually change the water to remove all traces of it. The positive side to using copper sulfate is that it will also kill the ich parasite if it is present. Consequently, you don’t need to distinguish between both parasites. Copper sulfate gets rid of all external fish parasites. Keep in mind that only the free-swimming form of the velvet parasite is affected by the copper sulfate – the encysted stage is not vulnerable to treatment. Another alternative is the use of products with acriflavine as an active ingredient; however, it may cause infertility.

Disease Prevention

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

  1. Provide the best environment for your fish to avoid stress on them. Educate yourself about any special needs a species may have.
  2. Maintain excellent water quality and perform frequent water changes.
  3. Monitor water quality weekly and keep a log to monitor changes.
  4. Avoid overcrowding.
  5. Feed a balanced and varied diet consisting of commercially prepared foods (e.g., flake, frozen, freeze dried), supplemented with live foods, and avoid overfeeding.
  6. Quarantine all new fish for a minimum of four weeks.
  7. Purchase fish from reputable sources.
  8. Observe your fish regularly to monitor changes in behavior or for symptoms of disease.
  9. Equipment shared between aquariums, such as nets, etc., should be soaked in water above 90 degrees Fahrenheit for several minutes or soaked in a weak bleach solution. Use liberal amounts of fresh water to remove any residual chlorine.


A quarantine aquarium is inexpensive to operate and can be set up easily. I suspect that most hobbyists have the necessary equipment. For the vast majority of freshwater aquarists, a 10-gallon aquarium with a sponge filter, heater, and short sections of PVC pipe or plastic 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. Some suggest keeping freshwater fish in saltwater (3 to 5 parts per thousand) during quarantine, as this will aid in eliminating any pathogens that are not salt-tolerant. In addition, it will help fish maintain their fluid balance. For the most part, I don’t use an antibiotic, but if a particular specimen shows early signs of disease, I will occasionally use a wide-spectrum antibiotic or formalin. The latter is also useful in eliminating protozoa.

So the next time you walk by your aquarium and see something amiss, you should be able to react quickly and make educated guesses as to how to treat your fish. Quarantine your fish, and perform regular partial water changes. Also observe your fish often to avoid these most common diseases in the first place.

Posted by: Chewy Editorial



By: Chewy EditorialPublished: