Common Aquarium Fish Illnesses and Symptoms

There are numerous ailments that can afflict our beloved aquatic pets. Without the benefit of veterinary equipment or training, it can be difficult to identify a specific illness seen in fish. We hobbyists need to rely on visual signs and our trusty water test kits to diagnose problems. In most cases, illness in fish can be traced back to stress from shipping, stocking, and insufficient water conditions.

Cory from Aquarium Co-Op recommends that every new fish should be quarantined and treated with three medications: General Cure, Ich-X, and Erythromycin. This buckshot method should cover you for almost any illness you are likely to encounter, caused by some pathogen or parasite. Of course this does not guarantee 100% survival of all new fish. Sometimes the trip from supplier to the store and eventually to your home aquarium is too stressful for some fish. But it does give you a big advantage for success.

Whenever using medication in your aquarium or quarantine tank, you should remove any carbon-based filtration. This type of filtration will actually remove the medication from the water, making it much less effective. After the treatment period is over, you can resume use of the carbon filtration.

Below is a quick list of some common freshwater ailments that you may encounter in your career as an aquarium hobbyist. I have arranged them in alphabetical order by common name.

Ammonia Poisoning

Symptoms: The symptoms for ammonia poisoning are similar to many of the other illnesses listed below. This is because ammonia poisoning can trigger many of the other illnesses on this list. Some common symptoms of ammonia poisoning include: gasping for air at the surface, rotting fins, loss of scales, clamped fins, lethargy, and jumping out of the water.

Causes: Ammonia poisoning is a result of an ammonia spike. Such a spike can be caused by excess food, lack of proper maintenance, a dead fish or other animal in the aquarium, or filter malfunction. It can also be caused by the sudden death of your beneficial bacteria colony. This could be caused by medication, improperly treated tap water, or lack of oxygen due to ceased water flow. In an enclosed system like an aquarium, it does not take long for ammonia levels to build up to a dangerous level.

Prevention and Treatment: The first and easiest way to prevent a build up of dangerous ammonia is to fully cycle your tank before adding fish to the water. When setting up a new aquarium, you should allow adequate time for your tank to “cycle.” This means giving beneficial bacteria time to colonize your filter and other surfaces of your tank. These beneficial bacteria will turn the toxic ammonia into progressively less toxic substances. You can learn more about this process by reading my article on the Nitrogen Cycle. You should also purchase a simple water test kit like the API Freshwater Master Test Kit. This will allow you to properly monitor the levels of many water important parameters in your aquarium. Ammonia should never read more than 0ppm, except when first cycling.

Your next step is to decide on a regular maintenance schedule and stick to it. An appropriate schedule is dependent upon the size of your tank, amount of food going into the water, and density of live plants. Removal of excess waste and uneaten food will dramatically reduce the amount of ammonia entering into the water.

In emergency situations, you should drain a significant amount of water from the aquarium and replace it with clean water. You can also administer emergency ammonia treatments like API Ammo Lock. These are temporary fixes, however, and you should find the source of your problem and deal with it accordingly.

Asphyxiation (Oxygen Starvation)

Symptoms: Gasping for air at the surface, jumping out of the enclosure, lethargy. Note: gulping air at the surface can be a normal behavior for some fish, like gouramis or bettas. If all of the fish in your aquarium are doing this (and they aren’t bettas or gouramis) then consider it a bad sign. You’ll need to take immediate action to prevent a fish kill.

Causes: In an aquarium, this can happen when there is too little water agitation. The direct result of this is that not enough air is being mixed into the water to allow oxygen to dissolve into the water. A secondary result of poor surface agitation is the growth of biofilm on the water surface. This barrier can grow thickly enough to prevent proper gas exchange from occurring by diffusion. Power outages are commonly the cause of sudden loss of water agitation and aeration.

A common cause of oxygen starvation in outdoor ponds is water temperature. Warm water is less efficient at holding onto dissolved oxygen than cold water. A pond that is receiving direct sunlight all day, especially in warmer weather, can easily become very warm. At about 86°F, water drastically begins to lose its dissolved oxygen. At that temperature, water holds half as much oxygen as water at freezing temperature. When night falls, the plants living in the water will stop photosynthesis but will continue respiration, using up more oxygen and producing more C02. This further aggravates the problem, potentially resulting in fish death.

Another potential cause of oxygen starvation is a sudden increase in the bioload of the aquarium. This can happen if a bloom of algae were to suddenly die and begin decomposing. Bacteria will quickly begin consuming the dead matter, producing a lot of CO2 and consuming oxygen, resulting in possible asphyxiation.

Prevention and Treatment: Simply make sure that your aquarium has sufficient water agitation and aeration through the use of filters, bubblers, and/or powerheads. Something as basic as an airstone connected to an air pump will provide all of the agitation you should need. In order to prevent the bloom of algae and the sudden death of that bloom, keep your aquarium well maintenanced with regular cleaning and water changes.

If you live in a region with frequent power outages, consider purchasing an electric generator for your home. If you only have a small aquarium, you can get by with a battery powered air pump.

Dropsy (Bloating)

Symptoms: Lethargy, lack of appetite, hanging awkwardly in the water, bloated appearance. The fish may bloat to the point of looking like a pinecone, because of its scales being extended outward as it expands.

Causes: Dropsy is not so much an illness as it is a symptom (like a fever in humans). There can be several potential causes of bloating, bacterial infection, swim bladder disorder, constipation, etc… Many of these causes can be traced back to poor water quality.

Prevention and Treatment: The best way to prevent something like this from happening is maintaining good water conditions for your pets. This means keeping a regular maintenance schedule.

Bloating caused by constipation can be prevented by feeding your fish sparingly and with a varied diet. Species which feed on plant material can be fed shelled peas, which are high in roughage. Carnivorous species like bettas or gouramis can be feed daphnia, which are considered a stool softener. If your fish is constipated, you can begin by halting feeding. If the problem does not go away, consider feeding one of the foods mentioned above.

Bloating caused by bacterial infection or swim bladder disorder can be impossible to treat if caught too late. If you catch the symptoms early enough, bring your fish to your local vet for advice and treatment.

Example of freshwater ich.
An Australian rainbow showing signs of an active ich infection.

Ich (Ick, White Spot Disease)

Symptoms: Small white bumps on the skin that resemble salt or grains of sand, clamped fins, gasping at the surface, rubbing against hard objects in the aquarium.

Causes: Ich is caused by a parasitic protozoa (Ichthyophthirius multifiliis). The white dots are actually individual parasites contained in cysts. It can easily be spread by an infected fish or aquarium gravel/water/decorations being moved from one tank to another.

Prevention and Treatment: Ich is treatable. If no attempt at treatment is made, the condition is 100% fatal. The parasite causes lesions on the skin and gills, which can lead to further infection and reduced ability to absorb oxygen. I recommend using Ich-X to treat an active infection as well as to prevent spread of the parasite during quarantine. You can prevent the spread of Ich by quarantining new fish and treating the water with a product like Ich-X to kill and parasites that may be infecting the new fish.

It is suggested that you increase the temperature of the water in your aquarium or quarantine tank. This increases the growth rate of the parasite, which allows the medication to work faster. This is because the medication only treats the parasites that are free-swimming in the water column.

Fin Rot

Symptoms: Torn or “melting” appearance to fins

Causes: Poor water quality and bacterial infection.

Prevention and Treatment: It’s easy to prevent fin rot. Simply maintain a regular cleaning schedule for your tank. Keep levels of ammonia and nitrate at 0ppm. If your fish shows signs of fin rot, it means you need to work harder to maintain a clean environment for your pet. An immediate water change and cleaning of your tank is necessary. If it isn’t too late, your fish will heal quickly if you maintain a clean environment for it.


Symptoms: White patches on the skin, resembling cotton fibers

Causes: One of a few varieties of water mold, which are commonly found in aquariums. They become problematic when a fish is already weakened from stress, injury, or another infection.

Prevention and Treatment: You can prevent fungal infections from becoming problematic by maintaining a clean, stress-free environment for your fish. The spores of this type of fungus take hold of a fish through open wounds or skin which is lacking in the fish’s natural mucus layer.

In order to treat an active infection, you should move your fish to a quarantine tank. This is because many treatments for fungus contain compounds that are harmful to invertebrates like snails and shrimp. Tetra Fungus Guard is a typical treatment. You might also want to use something like API Stress Coat to help repair any open wounds and loss of natural skin mucus.

Hole in the Head (HITH)

Symptoms: Pits or holes on the head and/or along the lateral line. If untreated, the lesions will increase in size, encouraging additional infections from bacteria or fungus. Left untreated, this condition is fatal.

Causes: HITH and Freshwater Head and Lateral Line Erosion (FHLLE) primarily affects cichlids, discus, and oscars. There is no singular definite cause of HITH, but there a few strong candidates: Hexamita parasites, nutritional deficiency, chronic stress, and poor water quality. Having two or more of these situations occurring in your tank indicates a high likelihood of this illness manifesting.

Prevention and Treatment: You can prevent this illness from occurring primarily by reducing stress in your fish, feeding a healthy diet, and maintaining clean water conditions. It is a good idea to properly quarantine and treat your newly arrived fish to prevent stress from parasites or other ailments.

If you have an active case of HITH or FHLLE, treatment means a buckshot approach, since the exact cause is hard to pin down. Firstly, you can treat an active Hexamita infestation with medicated foods like Hex-Shield by New Life Spectrum. Next examine your fish’s diet. Is it receiving all necessary nutrition? Research what your species of fish needs most and make sure it is receiving it. Lastly, consider tightening up your cleaning methods. Are you checking your water parameters? How often are you changing water? Is your filter functioning properly?

If caught and treated early enough, your fish should recover.

Columnare Disease (Flavobacterium columnare)

Symptoms: skin lesions, fin erosion and gill necrosis

Causes: bacterial infection, brought on by stress and poor water conditions.

Prevention and Treatment: Maintaining good water quality by doing 20% weekly water changes, including gravel vacuuming, and adequate biological filtration.

Nitrate Poisoning (Nitrate Shock)

Nitrate poisoning generally occurs in older tanks with infrequent maintenance schedules. Biological filtration in the aquarium converts ammonia into nitrite and nitrite into nitrate. Nitrates can only be reduced in the water by light plants, certain anaerobic bacteria, and most commonly through frequent water changes. I recommend changing 25% of your water, once a week to keep nitrate levels to a safe concentration. Prolonged exposure to elevated Nitrate levels can cause chronic stress, resulting in fish being more likely to succumb to other illness, like bacterial infections. When fish are moved from a tank of low nitrate concentrations to a tank with hight nitrates, or vice versa, this can result in “nitrate shock.”

Symptoms: Lethargy, loss of appetite, fin clamping, rapid breathing, laying at the bottom of the tank, curvature of body, death

Causes: lack of proper aquarium maintenance, over stocking, and poor water quality.

Prevention and Treatment: You can easily prevent complications from excess nitrates by doing weekly 25% water changes. You should also be careful not to over stock your aquarium and feed appropriate amounts of food. If you find levels of nitrates have begun to rise dangerously, conduct more frequent water changes in smaller amounts each time (for example, 5% to 10% a day until PPM are 20 or below). If your nitrates are in the hundreds of PPM, immediate and steady action is necessary. Change your water every few hours in small amounts (around 5%). Continue until your nitrates are again at a safe concentration.

Pick up a nitrate or general water test kit from your local pet store to monitor your tank parameters.

Nitrite Poisoning

Elevated levels of nitrites in the water  is highly toxic to fish. Nitrite prevents blood from efficiently transporting oxygen. Symptoms of nitrite poisoning can closely resemble those of oxygen deprivation.

Symptoms: rapid breathing, gasping at surface, lethargy, hanging out near filter outlet

Causes: lack of proper aquarium maintenance, over stocking, and poor water quality. Unfinished or interrupted nitrogen cycle

Prevention and Treatment: You can prevent nitrite poisoning by having a healthy nitrogen cycle established in your aquarium, and by performing regular water changes and filter maintenance. Lastly, do not overstock your tank, and in the case of new tanks, add livestock slowly. This gives your beneficial bacteria colony time to expand and accommodate the higher bio-load. If you are experiencing elevated levels of nitrite in your water (anything higher than 0PPM) you can begin treatment by doing a 50% or more water change. You should also increase aeration to compensate for your fish’s inability to use the available oxygen. Lastly, you can add one ounce of aquarium salt per gallon of water. The chlorine in the salt will prevent methemoglobin from building up in the fish’s blood.


Pop-eye is not itself a disease, but rather a symptom of illness.

Symptoms: One or both eyes bulging from the sockets, cloudiness in eye, blood in eye, rupture of eye

Causes: Physical injury, poor water quality, infection, incorrect water parameters

Prevention and Treatment: Because pop-eye is a symptom and not a disease, there is no one way to prevent or treat it. Good water quality and regular tank maintenance are the primary ways to avoid pop-eye. If your fish develops pop-eye, begin by doing a substantial water change of 50% or more. You could then use antibiotics like Erythromycin and aquarium salt to treat the possible underlying infection.


Symptoms: Concave abdomen, curvature of body, lethargy, erratic behavior, aggression

Causes: Infrequent feeding or incorrect food for fish diet

Prevention and Treatment: You can prevent starvation in your aquatic pets by establishing a sufficient and regular feeding schedule. The appropriate amount and type of food your fish needs depends on its age and species. Some parasites can cause digestive problems in fish, and may result in malnourishment. Parasites like Hexamita (that which causes HITH) can interfere with a fish’s ability to take up nutrients from food. You can treat Hexamita with medicated foods like Hex-Shield by New Life Spectrum.

Swim Bladder Disorder

The swim bladder is an important organ for fish. It is a gas-filled bag, which allows the fish to control its buoyancy. When problems with the swim bladder occur, a fish can have trouble maintaining proper orientation. Complications in the swim bladder can have many potential causes.

Symptoms: floating awkwardly, inability to reach the surface or to dive

Causes: high nitrates, parasites, constipation

Prevention and Treatment: Swim bladder disorder can be prevented by maintaining clean water conditions with proper feeding and tank maintenance. If the disorder is caused by constipation, many fish can be treated by feeding green peas.

Betta splendens, showing signs of Velvet infestation. Source:


Velvet is a fish disease caused by dinoflagellate parasites.

Symptoms: darting movement, scratching against objects in the tank, fin clamping, lethargy, dusting of parasites on the skin (green, brown, gold colored)

Causes: infected tank water, infected tools, weakened immune system, poor water quality

Prevention and Treatment: You can prevent an outbreak of velvet by conducting regular water changes and by sanitizing all tools and nets when moving between tanks. An active infection can be treated with medications containing copper sulfate, methylene blue, formalin, malachite green and/or acriflavin. Because the single-celled parasites are photosynthetic, you can increase the speed of treatment by keeping your infected tank in the dark.

Research Sources: – Ichthyophthirius multifiliis

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Featured Image Credits: Djpalme, Mydigitalife. Source:

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