17 Aquarium Fish Diseases – Guide, Symptoms And Treatment

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It happens to the best of us – you bring a fish home, things go well for a while, but then, without warning, one morning you find your new buddy listlessly swimming in the middle of the tank. Perhaps you see an odd growth or notice that your pet seems to have lost its appetite. What should you do?

Whichever form of illness your pet suffers from, the chances are you will fret and wonder what you should do to fix the problem. If you find yourself in this situation, this article is for you. Helping your fishy friend feel better can be a difficult task, but we’re here to help.

17 Common Aquatic Diseases

It is impossible to provide a complete ‘cure’ for every type of disease, as each condition has its own specific cause and symptoms. However, this article will give you a good overview of common illnesses and will give you the knowledge required to recognize symptoms when you see them.

From here, we hope that you can implement a suitable treatment plan and provide your pet with a speedy recovery. Without further ado, here is a list of 17 common diseases, along with their symptoms and recommended treatments.

Fish Tuberculosis (Fish TB)

Sick goldfish with bumbs on its scale, fish bowl pet

Fish Affected: Freshwater 

Cause: Bacterial infection

Symptoms: Weight loss, discoloration, presence of large bumps on the skin, and bulging eyes

Fish TB is a bacterial infection that occurs in freshwater fish kept in poor water conditions. The bacteria are found in small cysts that rupture after death and may infect healthy fish that feed on the flesh of deceased fish.

The best prevention method for this condition is to keep the aquatic environment clean. You should also quarantine any new arrivals before adding them to your aquarium.

This condition can also be cured with antibiotics such as streptomycin and isoniazid if caught early enough. Left untreated, the infection may spread throughout the entire body and cause death within 12 weeks after symptoms begin appearing.


Poecilia vivipara
Image Source : wikimedia.org

Scientific Name: Pterophyllum scalare

Fish Affected: Freshwater and Marine 

Cause: Viral or bacterial infection

Symptoms: Lying on the bottom of the tank, loss of balance, swollen belly with protruding fins

Dropsy is a disease caused by bacterial, viral, or parasitic infections. It may also result from a vitamin A deficiency. It is one of several common diseases that can affect virtually every species of freshwater and marine species. One of its most prominent signs is a swollen abdomen.

Sick fish suffer from inflamed kidneys that swell to twice their normal size, causing a bloated abdomen. This prevents them from draining fluids from the body cavity and internal organs, which then builds up and causes scales to stick outwards.

Dropsy can be prevented by providing a healthy diet rich in vitamins. You should also improve water quality and reduce stress as much as possible. You can use antibiotics to treat the condition if it is caused by bacteria. If the cause is unknown, then you should try treating it with antiparasitic medications after quarantine and observation of your pet for a few weeks.

Fish Pop-Eye

Dragon Eye Goldfish
Image Source: wikimedia.org

Fish Affected: Freshwater and Marine

Cause: Viral or bacterial infection

Symptoms: Bulging eyes that may look cloudy, loss of vision

Fish pop-eye is a common ailment caused by fluid building up within the eye. Common causes of infection include poor water quality, injury to the eye, and bacterial or viral infections. In some cases, one eye may be affected while in others both eyes are bulging and protruding from the eye sockets.

Pop-eye causes the infected to have difficulty seeing and swim in a zigzag or circular pattern in order to avoid obstacles. Pop-eye is difficult to treat because the pressure buildup inside the eye inhibits medications from reaching the site of infection.

To treat fish pop-eye, you should improve water conditions and use antibiotics to treat the condition if it is caused by bacteria. If the cause isn’t known, then you should administer antiparasitic medications after placing the infected individuals in a quarantine tank. Blind individuals are usually not affected since they do not rely on sight to hunt for food.

Fin Rot

Red Siamese Fighting Fish isolated on white

Fish Affected: Freshwater and Marine

Cause: Bacterial or fungal infections

Symptoms: Ragged fins and tail, loss of color

Fin rot affects all types of fish and occurs when the fins and tail become damaged. This damage may be caused by an injury such as fin nipping, or rough handling by humans. It can also be a result of a secondary infection caused by another type of disease.

Fin rot causes fins to appear inflamed, ragged, or frayed around the edges. As the condition progresses, small patches of decaying flesh are revealed where the flesh is eaten away. Fin rot results in death if not treated promptly because it can penetrate deep into the body cavity causing extensive damage.

You should treat fin rot by improving water conditions, reducing stress as much as possible, and administering antibiotics such as API Fin and Body Cure. Soak your poor little buddy in an aquarium salt bath containing potassium permanganate or copper sulfate for 5-10 minutes after quarantine and observation.


Showing sign of white spot on fish body.

Fish Affected: Freshwater and Marine

Cause: Parasitic infection

Symptoms: White spots on the skin, flashing, clamped fins

Ich is a very common disease that affects virtually all fish species. It is caused by Ichthyophthirius multifiliis, which are protozoan parasites that cause small white spots to appear on the. This disease is typically introduced by new arrivals that carry the disease.

Ich causes small white spots all over your pet’s bodies and fins. The skin may appear slightly opaque or have a metallic sheen to it, and they will rub themselves against objects in an attempt to stop the parasites from attaching to them.

The best way to treat ich is by improving water conditions, reducing the temperature of the aquarium water to at least 70 degrees F, and feed the tank’s inhabitants high-quality food. Treating with ich medications such as QuickCure or Rid-Ich can prove effective after the quarantine period is over.

Head and Lateral Line Erosion

Fish Head Song

Fish Affected: Freshwater and Marine

Cause: Bacterial infection, poor water quality, and parasitic infestation

Symptoms: Head is swollen and bumpy with indentations resembling a pinecone. The lateral line may have white eruptions that resemble cotton.

Head and lateral line erosion is caused by poor water conditions, bacterial infections, or internal parasites. Fish most commonly contract the illness when exposed to a sudden change in water conditions or from being kept in water that is too warm. 

This condition is marked by indentations on the heads, thickened scales on the bodies, and thickened fins. The lateral line, which is a series of sensory pores located underneath the head, may have white eruptions that resemble cotton.

Individuals with the illness should be treated for parasitic disease using an anti-parasitic medication such as Levamisole or Praziquantel. If the head and lateral line erosion is caused by poor water conditions, you should immediately improve water quality and provide a nutritious diet.

Gill Flukes

a close up picture of a fish gills

Fish Affected: Freshwater and Marine

Cause: Parasitic infection

Symptoms: Flattened gills that are red and swollen, clamped fins, difficulty breathing, gasping for air near the water surface

Gill flukes attach themselves to the gills and feed off blood. They can also penetrate the skin, teeth, and gills. Gill flukes are typically introduced by new arrivals to an established aquarium. Left untreated, it can cause death from anemia.

Symptoms of gill flukes include flattened red gills that are swollen and may appear dark brown or black in color because of blood stains. Clamped fins, breathing difficulties, and gasping for air near the water surface are all signs of gill flukes. To learn more about gill flukes, check out this extensive guide!

The best way to treat this condition is by performing a 30-40% water change and quarantining the infected fish in a hospital tank filled with a 2% salt solution. They should also be treated for internal parasites using an anti-parasitic medication such as Levamisole or Praziquantel.


Fish in the aquarium with plants

Fish Affected: Freshwater

Cause: Bacterial infection

Symptoms: Cotton like growths on the lips, eyes, and fins or large ulcers on the body

Sometimes known as cotton mouth disease, columnaris is commonly found among pond fish, or in tropical fish tanks that are poorly-maintained. This disease is highly contagious in poor water conditions because chronic exposure to high ammonia levels may leave your pet’s immune systems compromised.

Columnaris or flexibacter attack the skin of, forming small white ulcers that spread to the fins. As the infection worsens it begins to resemble cottony growths on the lips, eyes, and fins. Those affected by columnaris become listless and swim erratically near the surface of the water.

They should be treated as soon as possible because left untreated, they will die from organ failure and secondary infections such as fin rot and bacterial ulcers. The best way to treat this condition is by performing a 30-40% water change and administering an anti-bacterial medication such as penicillin or erythromycin.

Anchor Worms

Anchor Worms
Image Source : wikimedia.org

Fish Affected: Freshwater

Cause: Parasitic Infection

Symptoms: Short white “worms” that appear on the fins, mouth, and gills

Though they may look like actual worms, anchor worms are actually caused by a type of nasty parasite known as Lernaea cyprinacea. Anchor worms typically attach themselves to the fins, mouth, or gills and feed off their blood.

Anchor worms causes small white “worms” that appear on the fins, mouth, and gills. Though they aren’t visibly harmful to healthy individuals in your aquarium, they may cause serious damage to an already sick fish.

You will need to remove the anchor worms with tweezers or by hand if they are on the body of your pet. However, there is no recommended treatment for anchor worms because it will only lead to stress. Providing a nutritious diet while improving water conditions may help mitigate anchor worm infestations.

Ammonia Poisoning

ammonia chemical on the sink

Fish Affected: Freshwater and marine

Cause: Poor water conditions

Symptoms: Flashing, rubbing against objects in aquarium, loss of coloration, lethargy, rapid breathing.

Ammonia poisoning is a common disease in aquarium fish caused by excessive amounts of ammonia in the water. Ammonia spikes are often caused by uneaten food and dead organic matter decaying in the bottom of the tank. Over time, these harmful substances accumulate in the water and become poisonous to its inhabitants

Ammonia poisoning typically causes signs of stress such as flashing (rapid side to side motion), rubbing against objects in your aquarium, loss of coloration, lethargy, and rapid respiration. These symptoms indicate that the gills and skin are irritated and damaged.

One of the key treatment options is to start by performing a 30-40% water change, which will dilute the ammonia levels in the tank and eventually lead to optimal water conditions. Remove any uneaten food or dead organic matter. You will also want to add an ammonia reducer such as “Cycle” or Prime to your aquarium in order to detoxify harmful substances.


fish gill with saprolegniasis
Image Source: fisheries.tamu.edu

Fish Affected: Freshwater

Cause: Fungal infection

Symptoms: Small white cotton-like growths on the body, fins, or gills, loss of appetite

Saprolegniosis is a common fungal infection that is sometimes referred to as “cotton fin” and gets its name from the fungus Saprolegnia , which reproduces by producing small white cotton-like growths on the body, fins, or gills.

The fungus quickly spread through your aquarium water, infecting healthy individuals in the tank. Fish with Saprolegniosis are stressed out because of how much their skin is irritated by the fungus, which causes them to lose their appetite. Left untreated, Saprolegniosis will eventually lead to death

Therefore, if you notice any signs of body fungus, then it’s important to administer a fungal treatment, such as methylene blue or malachite green, to your aquarium. Place your pet in a quarantine aquarium with 10 gallons of water, and dose your tank according to the directions on the packaging.

Velvet Disease

Velvet Disease on a blue fish
Image Source: Instagram.com

Fish Affected: Freshwater and Marine

Cause: Parasitic Infection

Symptoms: Tiny white spots on the  skin, gills, and eyes

Velvet disease is one of the most common parasitic infections in fish. It’s caused by a microscopic organism known as Oodinium, which reproduces by infecting its hosts with itchy microscopic larvae. Marine species are infected by Oodinium ocellatum (causing a condition known as marine velvet or marine ich,) while freshwater species are infected by Oodinium catostomi.

Fish that are housed in dirty aquarium water are particularly susceptible because the spores of Oodinium are easily transported by the water current. The larvae of Oodinium attach themselves to the skin, gills, and eyes where they can live until they reproduce and infect others. 

The best way to treat Velvet disease is by using a velvet treatment such as Coppersafe or Cupramine according to the manufacturer’s dosage instructions. A dose of chloroquine phosphate is recommended especially if your pet has Velvet disease caused by Oodinium ocellatum, since it’s more effective on this type of parasite. 

Swim Bladder Disease

Sick goldfish swims upside down in aquarium.

Fish Affected: Freshwater and Marine

Cause: Bacterial infection and/or digestive issues

Symptoms: Difficulty maintaining equilibrium, gasping for air at the surface of the water

Swim Bladder Disease is a common condition in which the swim bladder (which controls equilibrium and buoyancy) becomes infected with or damaged by bacteria. This can be caused by an obstructed or blocked intestine in young betta and mollies who eat food with large unabsorbable particles, but the older ones can become infected too.

The symptoms of Swim Bladder Disease include difficulty maintaining equilibrium and gasping for air at the surface of your aquarium’s water. There is no known cure for this condition. Those affected will eventually recover on their own, but there are steps you can take to make sure that your little friend is comfortable.

Adding aquarium salt to your tank at a rate of 1 tablespoon per 5 gallons can help relieve some of the symptoms, while an adjustable flow nozzle is useful for making it easier for them to get air when they need it. You will also want to give them more hiding places so they have a place to rest & reduce their stress.

Lymphocystis (Cauliflower Disease)

Fish Affected: Freshwater and Marine

Cause: Viral infection

Symptoms: Raised white lumps on the fish’s skin that are usually circular or oval in shape. The skin may also appear darker or discolored near the white lumps.

Lymphocystis is a common viral infection that affects both freshwater and saltwater species. It’s caused by a virus known as either lymphocystivirus or hepadnavirus. Lymphocystis typically causes white lumps to appear on the skin, which can be circular or oval in shape with a raised bumpy texture, similar to cauliflower.

The good news is that Lymphocystis can’t infect humans or other animals, so it’s perfectly safe to handle your fish. It does not spread directly between the species but the virus can survive outside of a host for several days. This means that you can transfer the virus by handling an infected individual and then placing your hand on a healthy pet’s skin. 

There is no known cure for Lymphocystis, but it usually goes away on its own within 1 or 2 months and doesn’t cause any lasting damage. In some cases, it may last longer, in which case you may reduce the symptoms by maintaining a stable water temperature and adding aquarium salt to your tank at a rate of 1 tablespoon per 5 gallons.

Fish Lice (Argulas)

fish lice on the floor
Image Source: commons.wikimedia.org

Fish Affected: Freshwater and Marine

Cause: Parasitic infection by copepods

Symptoms: Raised red welts on the skin


Argulas is a parasitic infection caused by a small crustacean known as Argulus. They attach themselves to the skin of your pets and feed on their bodily fluids, causing them to develop raised red welts that look very similar to bites from a mosquito or fly.

These parasites are often introduced to an aquarium when new arrivals are added, but they can also be introduced through live food or plants. Early symptoms of Argulus look like itchy bumps, which turns into red welts over time. The longer you wait to treat this condition, the more stress it will cause and the more damage it will do.

The good news is that Argulus is easily treated with parasitic medications like QuickCure, but you should also remove any plants from your tank to prevent the parasite from laying eggs. Many fish are especially sensitive to medications, so you may need to repeat the treatment several times with a some time for recover in-between sessions

Vibriosis (Vibrio bacteria)

microscopic view of Vibrio bacteria

Fish Affected: Freshwater and Marine

Cause: Bacterial infection by Vibrio bacteria

Symptoms: White pimple-like growths on the skin that eventually burst to form raw open sores. The white blisters can also cause ulcers to form on the eyes.

Vibrio is a type of bacteria that causes open sores to form on the skin, ulcers to develop on the eyes, and can even cause your pets to lose their appetite. Vibrio is known as either vibriosis or septicemia depending on whether it develops in freshwater or saltwater species respectively.

The bacterium that causes Vibrio is found in both salt and fresh water and thrives during warmer months of the year when temperatures are between 75 and 85 degrees Fahrenheit, which makes it more common in summertime. It’s also commonly treated by adding aquarium salt to your tank at a rate of 1 tablespoon per 5 gallons.

Fish with ulcers on their face often lose their appetites, so you may need to soak their food in garlic or treat it with epsom salt or vitamin-C drops to improve the smell and taste of the food. Vibrio does not have a cure, but most cases are easy to treat provided that you start treatment early and maintain a stable water temperature.

Neascus (Black spot disease)

black spots on goldfish

Fish Affected: Freshwater

Cause: Parasitic infection by the nematode Acanthascaris acus.

Symptoms: Small dark-colored cysts on the skin and fins.

Acanthascaris acus is a type of parasitic nematode known more commonly as the black spot disease or Neascus cysts that infects the skin and causes dark-colored cysts to form on the surface of their body. 

Fish tend to contract the black spot disease after eating, but the larvae are usually introduced when an infected individual dies and rots in your tank. The cysts can grow quite large over time, but will eventually break open and release hundreds of tiny parasitic worms into your aquarium to start living off your other hosts.

To treat the black spot disease, you should regularly clean your tank by removing any decaying bodies to prevent the parasitic larvae from multiplying. You can also use parasitic medications like QuickCure to kill any remaining cysts or parasitic worms.

Preventing Aquatic Diseases

Prevention traffic sigh

Aquarium fish diseases can have a devastating effect on your fish population. Not only do some diseases spread quickly, but they can also be difficult to diagnose correctly. That’s why it’s important that you take the time to learn about different fish diseases and how they may affect your fish tank.

However, the best way to beat diseases like fin rot is to prevent them from happening in the first place! You can do this by following a few simple steps, which will also help keep your fish tank clean and healthy.

Buy From A Reputable Seller

The source of your fish is an important consideration. Ensure that your retailer of choice has a good reputation for providing healthy, disease-free animals. Check that they practice good husbandry and offer a guarantee on their livestock since this will help ensure that your new pets are healthy.

Quarantine New Arrivals

A view of a golden fish in a bag isolated on white background

When you bring new fish home, place them in a quarantine tank for two to three weeks before adding them to the rest of your aquarium inhabitants. This will give you time to observe the health of each individual, check for parasites or bacterial infection, and treat accordingly if need be.

Keep Your Tank Clean

Make sure to perform frequent water changes. If you are keeping tropical species, the entire tank should be cycled before anything else happens biologically. A new tank may take some time to reach proper biological balance but once this is achieved, it acts as a nice defense against water quality issues that lead to diseases.

Feed A Proper Diet

To keep your pet healthy, you need to feed them a high-quality diet. Ensure that the food you feed provides all the nutrients your little friend needs for good health, and resist the urge to feed them human-grade seafood or table scraps. This can cause them to become susceptible to illnesses, which you must avoid at all costs!


We hope this list of 17 common aquatic diseases helps you diagnose, prevent, and treat any diseases that you may come across in your tank. Please let us know if this list helped you diagnose or identify any of the illnesses above. If you found this listicle helpful, please share it with a friend!

In addition, it makes our day when readers like you leave comments with any additional information to help keep our pets healthy and happy. Do drop us a comment with your thoughts and questions – we’d love to hear from you!

Wanda is a second-generation aquarist from the sunny tropics of Malaysia. She has been helping her father with his freshwater tanks since she was a toddler, and has fallen in love with the hobby ever since. A perpetual nomad, Wanda does her best to integrate fish-keeping with her lifestyle, and has taken care of fish in three different continents. She loves how it provides a nice break from the hustle and bustle of life.

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