Aquarium Fish Diseases: Causes, Symptoms, and Treatments

aquarium fish diseases

Getting a pet fish can be one of the most fulfilling decisions you can make for your home. As a home aquarist, your wish is for your little inmates to be safe and healthy. It’s essential to be aware of various aquarium fish diseases. Hence, you have the right knowledge to help out your pets. Being ornamental fishes, they are susceptible to various pathogens. These pathogens are merely waiting for hosts to attack and kill. Thus, it’s crucial to act quickly and correctly when these diseases arise.

Aquarium fish diseases can be classified into bacterial, viral, fungal, or parasitic. Some of the aquarium fish diseases are not infectious in origin. Knowing how these different diseases are contracted and how they manifest in your fish might save them. To know more about common freshwater aquarium fish diseases and saltwater diseases in fish, just keep on reading. 

Table of Contents

Bacterial Aquarium Fish Diseases 

What are bacteria?

Bacteria are microscopic unicellular organisms that live in various environments. Our pet fishes are not an exception. While some bacteria are essential to your fish’s health, there are quite plenty of bacteria that cause aquarium fish diseases.

They are responsible for some freshwater aquarium fish diseases and a few saltwater aquarium fish diseases. Most bacterial diseases are susceptible to antibiotics. 


Infectious Agent

Signs and Symptoms

Bacterial Hemorrhagic Septicemia

Aeromonas hydrophila

Tiny Hemorrhages, abdominal distension


Vibrio anguillarum

External hemorrhage

Columnaris Disease (Cottonmouth)

Flavobacterium columnaris

Gray patches, cotton-like growth

Enteric Septicemia of Catfish (ESC)

Edwardsiella ictaluri

Oral hemorrhages, bulging eyes

Fish Tuberculosis

Mycobacterium marinum

Granuloma lesions, lethargy

Goldfish Ulcer Disease (Furunculosis)

Aeromonas salmonicida

Skin and gill discoloration, abscess on gills



Redness on mouth and fins, tiny hemorrhages

Bacterial Hemorrhagic Septicemia(BHS)

Aeromonas hydrophila is the bacterial species responsible for the fatal Bacterial Hemorrhagic Septicemia in aquarium fish. This particular bacterium targets both saltwater and freshwater species. BHS is usually treated with antibiotics. 

Signs and Symptoms of Bacterial Hemorrhagic Septicemia

Typically, Bacterial Hemorrhagic Septicemia targets the internal organs. Thus, visible hemorrhaging may be difficult to visualize. However, you can check for the fish’s skin for tiny petechial hemorrhages. Apart from this, internal bleeding would also manifest as abdominal distension or bloatedness. Furthermore, fish may also exhibit exophthalmos or bulging of the eyes. You can also see multiple ulcers in some parts of the body. Descaling or erection of scales is visible in most infected fish, as well.


Vibrio bacteria can also infect fish, as much as they like infecting humans. Vibrio anguillarum causes the dreaded Vibriosis in fish. It is also notorious for hemorrhagic septicemic disease in saltwater and freshwater species. Vibrio is a curved, rod-shaped bacterium with a small tail (flagellum) that helps it propel through the water. Furthermore, its primary target is the fish’s gastrointestinal tract or GIT.

Salmonid and Perciformens fish are commonly affected among marine fishes. Because of its characteristic red lesions, its former name was “red pest of eels.”

Signs and Symptoms of Vibriosis

Weight loss is one of the main signs of Vibriosis infection. You may also observe external bleeding all over the body, particularly the fins. These may be seen as tiny red spots on the front and sides of the fish. Furthermore, a distended abdomen and inflamed anus are very visible in the affected fish. It’s worthy of taking note that most fish die before showing any physical signs.

Prevention of Vibriosis

You can prevent Vibriosis through good water quality and low stocking densities. Slaughter or disinfection of infected fish is a good measure to help the spreading of Vibriosis in areas where it’s not endemic. 

Columnaris Disease (Cottonmouth)

The bacterium Flavobacterium columnare is the causative agent of the Columnaris Disease. It is also called tail rot, cotton wool, mouth fungus, or mouth rot. The bacterium is named as such due to its columnar shape. It’s one of the most prevalent freshwater aquarium fish problems. Generally, the bacterium is found in virtually all aquariums. However, they only present with symptoms when the fish’s immune system is weak. Salmonids and catfishes are among the most often targeted species. The incubation period is only short, ranging from 24 hours to 48 hours in most fishes. 

Signs and Symptoms of Columnaris Disease (Cottonmouth)

The first sign of the disease is the appearance of gray patches on the dorsal fin area. This lesion is the characteristic “saddleback” lesion. Moreover, the lesion may progress to the point of erosion, which eventually exposes the muscle tissue to the environment. 

This disease is remarkable for its specific characteristic – cotton-like growth near the mouth. Typically, the disease is wrongfully diagnosed as a fungal infection due to cotton growth. Moreover, white spots all over the body also appear. On the fin, saddle-like lesions may also appear. 

Diagnosis of Columnaris Disease (Cottonmouth)

Columnaris Disease is diagnosed by examining the fin scrapings. Under the microscope, the lab technician will look for the specific bacterium that’s columnar to confirm the diagnosis. Furthermore, vets will obtain a bacterial culture in some cases, as well. 

Prevention of Columnaris Disease (Cottonmouth)

Quarantine any new fish you might introduce to your aquarium. It is to avoid infecting your old existing fishes in the aquarium in case your new fish might be carrying the bacterium. As previously mentioned, always keep your fish’s immune system at its best. Make sure they’re not exposed to extremely high temperatures. Moreover, avoid crowding the aquarium to avoid putting them in a great deal of stress.  

Enteric Septicemia of Catfish (ESC)

Enteric Septicemia of Catfish is more popularly called the Hole in the Head Disease. Other names are green knife fish and walking catfish. It is caused by Edwardsiella ictaluri. Studies suggest that virtually all internal organs become infected with the bacteria in the later stage of the disease. Particularly, kidneys suffer from severe illnesses. Furthermore, discolored lesions also appear on the liver and spleen. Infected fish include catfishes, Japanese eel, as well as knife fishes

Signs and Symptoms of Enteric Septicemia of Catfish (ESC)

Initially, infected fish present with petechiae or tiny hemorrhages on the mouth area, throat as well as the fins. These hemorrhages often progress to ulcers. Other signs and symptoms may include bulging eyes (exophthalmos). Moreover, skeletal muscles may also have hemorrhages. You may also observe pale gills in infected fish. 

In the severe stages of the disease, you may also observe a swollen head among fish who are infected. It will often progress to ulceration, which will cause the “hole in the head.” Apart from the whole, the brain is also inflamed as well as the meninges. 

Prevention of Enteric Septicemia of Catfish (ESC)

The bacterium is transmitted through the fecal-oral route and exposure to contaminated water or aquarium materials. Thus, maintaining the cleanliness of the fish’s environment is crucial in the prevention of ESC. Those who have managed to survive the disease remain as carriers. Therefore, quarantine of new fish and recovering fish should be observed. Lastly, ensure your fish’s immune system is working at its best. An intact immune system is key to avoiding any aquarium fish diseases.

Fish Tuberculosis

Surprise! It’s not just us, humans, who get infected by cancer-like Tuberculosis. Even fish get infected by Mycobacterium and cause tremendous freshwater diseases in fishes. It’s often referred to as the Silent Killer of the aquarium for its gradual but extreme effects on the fish’s health. Bettas, Siamese Fighter, and Gouramis are among the most commonly affected types of fish. However, Mycobacterium marinum can infect other kinds of fish as well.

aquarium fish diseases

Signs and Symptoms of Fish Tuberculosis

The granulomatous disease is the unique characteristic of tuberculosis infections. Granulomas appear in the form of white growths or lesions. Listlessness and anorexia are among the most common symptoms of Fish TB. Fin erosion is a common external manifestation, as well as reddened skin and the appearance of colored bumps. Furthermore, bloatedness and swelling of any part of the body may be visible. Sunken eyes and forehead are some of the things to watch out for. It indicates that TB has invaded the head and brain of the fish. 

Treatment of Fish Tuberculosis

Just like in humans, treatment of Fish TB is a little bit more tedious than other bacterial infections. Fish TB isn’t just treated with one particular antibiotic. Usually, treatment requires a combination of drugs to eradicate the disease. Furthermore, Fish TB treatment requires a long time. 

Can I contract Tuberculosis from my fish?

Fish TB is unique in most aquarium fish diseases because it’s highly contagious. Some pet owners may even be concerned if they can acquire TB from their fish. Mycobacterium tuberculosis is the species that causes TB in humans. However, in fish, it is Mycobacterium marinum. However, there are instances when Fish TB can be zoonotic (jumps from animals to humans). When human skin comes into contact with Fish TB lesions, they may get infected and present with skin infections. Thus, if you suspect your fish to be infected, always wear protective gloves when you’re handling the aquarium.

Prevention of Fish Tuberculosis

Let’s face it. Fish TB is very difficult to treat. Sometimes, the disease gets very bad that the fish doesn’t recover fully. Thus, as prevention and precaution, others euthanize their fish. If you don’t want to do this, you could do a few things to prevent Fish TB.

Remove the sick fish from the tank. Clean the tank completely and make sure to sterilize everything, including the aquarium tools and other materials. Otherwise, the quarantine won’t be any effective. During this period, observe the sick fish as well as the uninfected ones for symptoms. 

Prevention of Fish TB relies on making sure your fish are healthy. Make sure they are well-nourished and are not exposed to stressful situations. Furthermore, make sure anything you’re introducing to the aquarium is not contaminated. 

Goldfish Ulcer Disease (Furunculosis)

Aeromonas salmonicida is responsible for a notorious disease among koi and goldfishes. It causes furuncles or multiple boils on the skin of the fish. Moreover, furunculosis may also cause ulceration of internal organs. Apart from these species, Aeromonas also targets other saltwater and freshwater fish species. It’s also called carp erythrodermatitis. Furthermore, this disease primarily affects GoldfishcarpsJapanese eel, among many others. 

Signs and Symptoms of Goldfish Ulcer Disease (Furunculosis)

Signs and symptoms usually differ depending on the age of the fish. However, some of the symptoms occur in both age groups. A fry often presents with discolored skin and fins, hemorrhaging of muscles, exophthalmos, or popeyes. Hemorrhages are visible in the fins and gills of fish. Later on, this will develop into craters and abscess that release thick, pus-filled discharge. Furthermore, young fish usually experience an increase in respiration. 

Adult fishes present with the cessation of feeding or anorexia. Concurrent with this is weight loss. Moreover, they would also present with lethargy and paleness. Edema or swelling is visible in most cases. It indicates swelling of internal organs like the spleen and liver.

Note that when young fish are infected, the progression is often quick. It often results in the death of sick fish within 2-3 days. 

Prevention of Goldfish Ulcer Disease (Furunculosis)

The best way to prevent Goldfish Ulcer Disease in any kind of fish is to quarantine any new fish before bringing them into your aquarium. Ensure the aquarium is free from any sharp objects that may cause abrasion or wounds on your fish. Note that open cuts and scrapes are the points of entry of the bacteria. Salt is prophylactic against some aquarium fish diseases. Lastly, when buying new fish, make sure you’re buying healthy stock. 


Pseudomonas fluorescens is another bacterial genus that causes septicemia or infection of blood in fish. This bacterium is not selective when it comes to their target. Virtually any fish species are susceptible to the infection. Typically, a low immune system is a predisposing factor for getting Pseudomoniasis.

Signs and Symptoms of Pseudomoniasis 

Redness around the fins and mouth area are both signs of ongoing septicemia in fish. It is also visible in the fish’s anal region. Moreover, you’re more likely to see petechial hemorrhages (tiny red dots) on the fish’s ventral surface, which is a sign of ongoing internal bleeding. Other symptoms include belly swelling and eye bulging.

Parasitic Aquarium Fish Diseases

Parasites are the ickiest creatures to infect your fish. They can be either protozoan (microscopic multicellular organisms) or worms. Parasites are multicellular organisms that can either be microscopic or visible to the naked eye. These creatures live off of their host’s nutrients and normally won’t survive long without them. 

Fish serve as host to quite a variety of parasites. It might be helpful to know some of the most common ones that might latch on to your fish. 


Infectious Agent

Signs and Symptoms

Brooklynella hostilis (Clownfish Disease)

Brooklynella hostilis

Abnormal swimming, rapid breathing

Trematodes (Fluke)

Trematodes or flatworms

Redness of gills, scratching against objects

Nematode (Roundworm)

Nematode or flatworm

Belly swelling, inflamed anus

Ichthyophthirius multifiliis (Ich)

Ichthyophthirius multifiliis

White spots on skin



No new fish, dead eggs

Anchor Worm

Lernaea crustaceans

Green or red worms on skin/wound

Velvet Disease


Brown-gold, velvety skin/scale

Tetrahymena (Guppy Disease)


White spots, ragged fins

Neon Tetra Disease

Pleistophora hyphessobryconis

Visible cysts

Brooklynella hostilis (Clownfish Disease)

Brooklynella is a parasite that infects marine aquariums. Its name comes from the city of Brooklyn due to the massive outbreaks of the disease in the area. Some often refer to this as the Clownfish Disease or the Slime Disease for its characteristic slimy gills. These ciliated parasites feast on fish’s dead skin cells, particularly their gills, which cause severe damage to their gills, which often affects their respiration. In some infected fish, death can occur within just hours from the infection.

Signs and Symptoms of Brookylynella hostilis (Clownfish Disease)

Manifestations of Clownfish disease include rapid respiration, abnormal behavior, or swimming, as well as slim on their gills. These slimes often plug their gills, making them difficult to breathe. Moreover, infected fish will also present with some lethargy. 

Treatment of Brooklynella hostilis (Clownfish Disease)

Treatment of Clownfish Disease is varied. Most fish owners use Formalin to get rid of this parasite among aquarium fishes. Most use a 37% formalin solution and use it as a bath for the fish. However, this might be difficult to procure or prepare on your own. 

Prevention of Brooklynella hostilis (Clownfish Disease)

Maintenance of your aquarium is the best way to prevent Brooklynella from pestering your fish. Avoid bringing in wild clownfish to your aquarium because most of them carry this parasite. Otherwise, put your new fish in quarantine first before mixing them with other fish in the aquarium to avoid spreading the parasites.

Trematodes (Fluke)

Flukes are annoying parasites that cause irritation and sometimes harmful damage to the fish. They are commonly called flatworms. Flukes use fish as hosts to get nutrients from as well as a place to reproduce. Unfortunately, most flukes are too small to be seen by the naked eye, unlike other worms.

These are dangerous parasites that are harmful to humans. You may be familiar with them because they cause lethal diseases in humans in the form of undercooked fish. They are some of the common saltwater aquarium fish problems you may encounter.

Signs and Symptoms of Trematodes (Fluke)

Typically, a certain number of flukes won’t cause any danger or harm to your fish. However, when the number exceeds, it might cause illness to your fish. Signs and symptoms include redness, especially over the gills. Moreover, you can also observe occasional bruising. Infected fish may also produce excessive slime as a reaction to the parasite. However, because it’s excessive, the slime becomes harmful to the fish because they end up suffocating. 

Fish may also scratch themselves against objects in the aquarium to get rid of the parasite. However, this only causes abrasions and cuts to their scales, making them prone to secondary bacterial infections. 

Treatment of Trematodes (Fluke)

Thankfully, trematodes or flukes are much easier to treat as opposed to other types of diseases. Your vet will most likely prescribe some antiparasitic medications like Praziquantel. Along with this, constant water change in the aquarium is necessary to help avoid the recurrence of the disease.

Nematodes (Roundworms)

Perhaps the most well-known parasite among aquarium fishes is roundworms. These worms are cylindrical and quite longer than flatworms and less flat than flatworms. They are quite big enough to be seen by the naked eye, and they just look disgusting! 

Most of the time, roundworm parasites are not harmful or lethal. However, when young fish become infected, it becomes deadly. Capillaria is an example of a roundworm that usually resides in Angelfish species. This particular roundworm is fatal when ingested by humans. 

Another roundworm is Camallanus (red roundworm). It usually infects eels and other ornamental fishes in aquariums.

Signs and Symptoms of Nematodes (Roundworms)

Manifestations of roundworm infestation greatly depend on the site of infection. Usually, these worms reside in the gastrointestinal tract. However, they can practically live anywhere in the fish’s body. In the early stage of GIT infestation, fish might have some abdominal distension or bloatedness. As the number of worms becomes extremely high, you may notice the belly swell even more. Meanwhile, in some fish, roundworm infestation may cause wasting despite adequate food. You may also spot inflammation and lesions anywhere in the body. 

In Camallanus roundworms, you will see red roundworms coming out of the anus of fish. Usually, people mistake this for fish feces. Along with this, the fish’s lower body may appear wasted. Eventually, the whole body will look wasted because the worm is stealing the nutrients from the fish. 

Prevention of Nematodes (Roundworms)

One thing to take note is that nematodes or roundworms lay eggs. Thus, to eliminate the disease and avoid reinfection, make sure to rid the aquarium of the eggs. Otherwise, the condition will just recur. 

To actively treat the parasitic infection in fish, antihelminthic drugs will be prescribed. These include Praziquantel

Treatment of Nematodes (Roundworms)

Fish owners typically use potassium permanganate to get rid of Anchor Worms quickly. It is used as a fish dip or bath to remove the Anchor Worms on the fish easily. Adding salt to the aquarium also prevents secondary bacterial infections.

Ichthyophthirius multifiliis (Ich)

Some people call it Ichthyophthirius multifiliis or Ich the White Spot DiseaseIch is one of the most common freshwater aquarium fish diseases. Unfortunately, this disease is highly deadly among aquarium fish. As the temperature increases, the ciliated parasite’s activity increases. The good thing with this disease is when fish have been previously infected; they develop some form of immunity against future reinfections.

Signs and Symptoms of Ichthyophthirius multifiliis (Ich)

White spots are the defining characteristic of Ich. These are located on the body or the fins. These tiny white spots are called trophonts, which later increase in size. These often look blister-like. Furthermore, if the gills are infected, sick fish may have a hard time breathing. 

Prevention of Ichthyophthirius multifiliis (Ich)

Observation is one of the easiest and most effective ways to prevent Ich. Look for aberrant behavior in your fish. If you spot Ich as soon as it attacks, the chances of it being deadly decreases. When bringing in new fish, put them in quarantine for at least two weeks. 

Treatment of Ichthyophthirius multifiliis (Ich)

Treatment of Ich is quite tricky. The fish’s immune system does something unique when infected by the parasite. Once the ciliated parasite latches on the fish, the fish does a “walling-off” effect to contain the injury. This encapsulation process makes it difficult for drugs to penetrate within the infected area. 


Planaria is a flatworm that’s widely popular to cause indirect harm to some fish species. These worms don’t usually live inside a fish’s body. However, they feast on fish eggs. Thus, it’s a complete annoyance if you’re trying to breed some species of fish that lay eggs. What’s unique about Planaria is that it’s asexual. They can multiply without mating with other Planaria. This indicates that they multiply quite quickly. Furthermore, Planaria infects both saltwater and freshwater environments.

Signs of Planaria

Because Planaria doesn’t directly affect adult fish, symptoms are not there to spot the parasite. Signs that your aquarium might contain Planaria are when your fish aren’t increasing in number. Because the flatworm feasts on the eggs, no new fish are being born over time. 

If you wish, you can look for the worm with the help of a magnifying glass. They have an easily distinguishable triangular head. Furthermore, they are white (sometimes brown) in color and also have cross-eyes. 

Anchor Worm

Despite the worm in its name, Anchor Worms aren’t worms. They are crustaceans belonging to genus Lernaea. They are copepods that burrow in a fish’s skin. Typically, manually removing the worms is the prevention of the disease. Anchor worms mainly target koiGoldfish, and other carps

Signs and Symptoms of Anchor Worm

The good thing about Anchor Worms is that they are quite visible even without a magnifying glass. These worms are usually red or green seen within a wound. Signs of infestation include redness or inflammation in the area where the Anchor Worms are buried. 

Velvet Disease

Velvet is a disease that causes fish to appear brownish-gold or rusty. For this, it is sometimes called Rust Disease. The causative agent is a dinoflagellate belonging to the Piscinoodinium genus. Lungs and skin are the primary targets of this dinoflagellate parasite. Copper is the most effective treatment for Velvet Disease.

Signs and Symptoms of Velvet Disease

In mild diseases, fish will show some irritation and inflammation in their fins and gills. Eventually, the condition will progress and will lead to bleeding. Typically, lesions will appear brownish-gold and have a velvety texture. Once the lungs are attacked, fish will experience rapid breathing. If the lungs fail to supply oxygen to the body, fish will show suffocation signs. 

Tetrahymena (Guppy Disease)

Tetrahymena is a free-living protozoan that causes Guppy Disease. They mainly like to target guppies. This parasite lives inside the fish’s scales, skeletal muscles, as well as internal organs. Despite them being parasites, they can live in the aquarium for a long time without hosts. Moreover, they can also infect other types of fishes. 

Signs and Symptoms of Tetrahymena (Guppy Disease)

Unfortunately, the ciliated parasite is too small for fish owners to see without magnifying glasses or microscopes. Affected fish will have white spots in various parts of their bodies. Sometimes, it might be mistaken for the White Spot disease. Furthermore, their fins are also affected. They will appear ragged. Fish’s scales will also appear rough. Other systemic diseases include lethargy and breathing difficulties. 

Neon Tetra Disease

Pleistophora hyphessobryconis is the culprit for the Neon Tetra Disease. It is a microsporidian – a protozoan. Most aquarium hobbyists are quite familiar with this nasty disease that infects Neon Tetras and any type of fish. This parasite eats the fish from inside out. 

aquarium fish diseases

Signs and Symptoms of Neon Tetra Disease

Visible cysts are present in infected fish. Along with this, your fish’s scales may lose some color and appear pale. Because of the cysts in the body, the fish may have difficulty swimming. Moreover, as the disease worsens, your fish will just look very ill and restless.

This disease is one of the most dreaded aquarium fish diseases. This is because there is no cure for it. Thus, most fish breeders are left with no choice but to euthanize sick fish to avoid spreading it to others.

Fungal Aquarium Fish Diseases

Fungi are multicellular organisms that appear as cotton, spores, thread-like, and other forms depending on their state and species. Molds are also a type of fungi. It might not seem like it, but fungi are among the most prevalent aquarium fish diseases you will encounter. These organisms are quick to reproduce and can spread throughout the aquarium.


Infectious Agent

Signs and Symptoms

Saprolegnia (Water Mold)


Fuzzy patches on skin

Branchiomyces (Gill Rot)


Rapid respiration, difficulty of breathing

Saprolegnia (Water Mold or Egg Fungus)

The fungi Saprolegnia or water mold is well-known because it takes advantage of sick fish. These fungal infections are often mistaken as one of the viral freshwater aquarium fish diseases (Lymphocystis). It is an opportunistic fungus that reproduces when the fish’s immune system is at its lowest. These particular bugs are fond of eating dead skin cells off of sick fish. Fish eggs are also one of the main targets of the Water Mold. For this, they are often called Egg Fungus.

Signs and Symptoms of Saprolegnia 

The characteristic feature of Saprolegnia is fuzzy patches on the fish’s skin. These may often look like cotton on the skin. When the internal organs are infected, necrosis and hemorrhage of the organs may occur. It is seen as bloating in fish, especially the young ones. There may also be lifting of scales present in the fish.

Prevention of Saprolegnia 

Optimum water quality is one of the essential things to observe to prevent the water mold from reproducing in your aquarium. Secondly, regularly check your aquarium’s temperature to make sure it’s not getting colder than it’s supposed to be. This is to prevent creating a favorable environment for the fungus. Lastly, the fungus reproduces in immunocompromised hosts. Thus, to prevent this, boost your fish’s immune system by avoiding stress, proper nourishment, and healthy water.

Branchiomyces (Gill Rot)

Gill rot is a relatively uncommon but fatal fungal disease in aquarium fish. Typically, these fungi reproduce in high nitrate-containing aquariums. The fungi most specifically target the gill tissues causing a great deal of respiratory distress in those affected.

Signs and Symptoms of Branchiomyces (Gill Rot)

Difficulty in respiration or breathing is the most common sign of Gill Rot. Moreover, gills would also appear rotten and inflamed. Patchy marbled-gills is a definitive sign of the fungal disease. Because it is so severe, infected fish may die within 48 hours without treatment. 

Viral Aquarium Fish Diseases 

Viral infections or diseases aren’t common for saltwater aquarium fish diseases compared to other types of aquarium fish diseases. What’s even striking about this group of diseases is that it’s difficult to spot because symptoms are rather unspecific. Currently, there is no specific or targeted treatment for viral infections. When your fish has it, we are only left to wait for the fish to fight off the infection and survive it. In some cases, fish may receive supportive treatment. 

What are viruses?

Viruses are small, non-living things, made up mostly of genetic material. These little bugs are usually a lot smaller than bacterial cells. The tricky thing about them is that they are quick to spread and are generally highly contagious. They make use of the animal’s living cells to replicate and do this in such a rapid manner. 


Infectious Agent

Signs and Symptoms

Carp Pox

Herpesvirus cyprini

Gelatinous, white nodules

Koi Herpesvirus


Dry fins, behavioral changes

Angelfish Virus Disease

Angelfish virus

Excessive slime



Wart-like nodules

Viral Hemorrhagic Septicemia

Psicine novirhabdovirus

Bloatedness, bulging eyes, tiny hemorrhages

Infectious Pancreatic Necrosis

Infectious Pancreatic Necrosis virus

Bloatedness, bulging eyes, disorientation

Carp Pox

Carp Pox is one of the most common freshwater aquarium fish diseases. Herpesvirus, particularly Herpesvirus cyprini, is responsible for this disease. They mainly target the blood cells, kidney, liver, and other major organs. This virus targets mostly koi species, among other fishes.

Signs and Symptoms of Carp Pox 

Carp pox typically presents with gelatinous white, creamy, or milky-white plaques on the fins. These are often candle wax-looking. Unlike other skin lesions, these plaques often increase in size over time. Moreover, most of these lesions become tumors in the long run. However, they are not fatal. Usually, what you should watch out for are secondary bacterial infections that may arise. These are often what make this disease fatal.

Prevention of Carp Pox

As with most Herpesvirus infections, the virus remains latent in the fish without showing any signs and symptoms. However, when the fish’s immune system hits a drop, the virus gets an opportunity to attack, and the fish starts showing symptoms. Thus, it’s best always to make sure your fish’s immune system is at its optimum. 

Furthermore, making sure that you’re creating a stress-free environment for your fish is also crucial. You can do so by ensuring the aquarium’s temperature is suitable. Also, giving them the right food at the right time. If you want to help your fish, even more, try giving some immunostimulants to help boost their immune systems. 

Koi Herpesvirus

The Koi Herpesvirus is a lot like the Carp Pox virus, although different virus species causes it. Cyprinid herpesvirus 3 is the infective agent of the disease and mainly targets ornamental carps. Furthermore, it is among the many freshwater aquarium fish diseases. This particular virus targets mostly the gill tissues and causes inflammation and necrosis. 

Koi Herpesvirus’s typical incubation is around 8-21 days. However, according to studies, naive fishes manifest symptoms as early as three days. 

Signs and Symptoms of Koi Herpesvirus

Signs and symptoms of this virus include lethargy, fish swimming on the surface of the water, and some hemorrhages all over the body. Moreover, sunken eyes are also present in infected fish. Fin adhesions are also seen in most infected fishes. Along with this, dry fins is one of the most remarkable signs. Behavioral signs include lethargy and increased respiratory rate. In contrast, some fish might show signs of disorientation and hyperactivity. 

Angelfish Virus

The Angelfish virus is targeting the species of Angelfish and causing devastating effects on their health. It has a 3-week incubation period during which the fish won’t have any symptoms but can infect others. It is such an aggressive and contagious virus that even if an angelfish survives it, the fish remains to shed the virus for up to 6 months after the disease. 

aquarium fish diseases

Signs and Symptoms of Angelfish Virus 

Some of the most commonly occurring signs and symptoms are excessive slime production and fish sinking at the bottom of aquariums. Moreover, some fish exhibit folding of fins onto the body when infected. For some unexplained reason, these fish may also have noses that are pointed up. Despite its quite long incubation period, most fish start getting very ill within just three days.

Prevention of Angelfish Virus 

Firstly, it’s essential to identify if there’s a disease in any of your fish because it’s the first step to preventing its spread. It applies to other aquarium fish diseases as well. You can’t always make the right diagnosis at home. In some cases, you might need your veterinarian’s professional advice to make a definitive diagnosis.

Lymphocystis Disease

Iridoviruses are responsible for Lymphocystis Disease. They target both freshwater and saltwater fish species. This virus mainly targets the fish’s connective tissues causing them to have nodules on their fins. However, this disease isn’t fatal. They could be bothersome to the fish and may interfere with eating and nutrition if lesions are on the mouth region. 

Signs and Symptoms of Lymphocystis Disease

Signs and symptoms include wart-like nodules or lesions on the fins, gills, and oral and eye areas. These lesions can be in different colors (white, pearl, cream, pink, brown, dark brown). The healing process usually takes up to 9 months. Thus, though it may not be life-threatening, it could be inconvenient for your fish.

Who gets Lymphocystis Disease?

According to studies, the virus causing Lymphocystis usually infects clownfishesgobiessunfishes, and other bony fishes. Lymphocystis-causing Iridoviruses rarely infect other fish like Goldfish, koi, catfishes, etc.

Viral Hemorrhagic Septicemia

Viral Hemorrhagic Septicemia or VHS is one of the most dreaded saltwater diseases in fishPsicine novirhabdovirus causes this disease. The virus may infect freshwater fishes as well. Usually, this virus attacks Salmon fishes but may infect trout, flounder, and other fish. Moreover, the virus’s incubation period is around 1-4 weeks depending on the temperature (1-2 weeks for warm, 3-4 weeks for cold). 

Signs and Symptoms of Viral Hemorrhagic Septicemia

What is alarming about the disease is its lack of external manifestations. Hemorrhage or internal bleeding usually occurs internally. Thus, it is unlikely that you could see any signs and symptoms directly related to bleeding. However, signs like bulging eyes (exophthalmos) and bloated abdomen may signify an ongoing hemorrhage. 

Moreover, you may also watch out for lack of appetite or anorexia. It is helpful to observe their swimming behavior because infected fish usually show changes. In some cases, tiny hemorrhages on the fish’s eyes may be visible. As with other diseases, carrier fishes may show no symptoms at all.

How is Viral Hemorrhagic Septicemia Transmitted?

The Viral Hemorrhagic Septicemia virus is mainly passed on through the fish’s urine and reproductive fluids. They mostly enter the body through the gills or an open wound or skin abrasions. 

Infectious Pancreatic Necrosis

Infectious Pancreatic Necrosis or IPN is a lethal disease that affects mainly freshwater fishes, particularly trout and salmon. The virus belongs to the Birnaviridae family. Unfortunately, the disease has a high mortality rate and currently has no available cure. This disease is also more common in younger fish and less frequently seen in adult ones. 

Signs and Symptoms of Infectious Pancreatic Necrosis

Affected fish’s color may be darker. They may also present with a bloated abdomen and bulging eyes. Tiny hemorrhages may also be seen on the fins and the body. Furthermore, their swimming movement may also be erratic. Terminal symptoms include poor respiration and abnormal, jerking movements. 

Prevention of Infectious Pancreatic Necrosis

Currently, there is no proven way to prevent Infectious Pancreatic Necrosis through vaccination or other medicines. Make sure you’re propagating IPN-free eggs in uncontaminated water. Furthermore, it’s helpful to maintain the aquarium’s temperature at no less than 7˚C to avoid the virus. Some studies show that chlorine is effective against the virus.

Other Aquarium Fish Diseases


Infectious Agent

Signs and Symptoms

Ammonia Poisoning


Gasping for air, discolored gills

Swim Bladder Disease


Swimming on side or upside down



Abdominal swelling, eye bulging

Malawi Bloat


Belly swelling, loss of appetite

Ammonia Poisoning

Ammonia is a compound that contains nitrogen and hydrogen with the chemical formula of NH3. It’s considered poison when a fish ingests too much of it. The most common poison’s effect is on the fish’s gills. You could see a difference in the color of the gills. Typically, they would turn reddish or purplish. Moreover, you’d also notice the fish gasping for air at the aquarium’s surface due to gill damage. Eventually, the fish will experience lethargy as the poison continues to damage the organs. 

aquarium fish diseases

Prevention of ammonia poisoning is through the establishment of a healthy aquarium. Make sure the aquarium contains gravel at its bottom. These gravels are houses to the bacteria that make use of nitrogen in their life cycle. It is to ensure that nitrogen circulates properly in the aquarium and doesn’t get built up. Furthermore, avoid overstocking the aquarium as well. This will help prevent not only ammonia poisoning but other infections, too.

Swim Bladder Disease

Despite it being a disease, Swim Bladder Disease isn’t a disease but a symptom. There is no one specific cause of having a swim bladder. The swim bladder is an organ that helps fish maintain buoyancy.

You can easily spot a fish with Swim Bladder Disease. The fish would swim on its side, sometimes even upside down. It is because they are unable to regulate the air coming in and out of their bodies.

There are plenty of things that cause Swim Bladder Disease. Constipation is the most common cause of a Swim Bladder. Goldfish usually have this condition. Other causes include bloatedness from parasitic infection. Foods that ferment in the stomach may produce too much air in the gut, causing Swim Bladder Disease. 


Dropsy or edema is a common aquarium fish disease. It is due to the build-up of excess fluid in the fish’s abdomen and tissues. Just like Swim Bladder Disease, dropsy is not a disease but a symptom. Common causes of dropsy are bacterial infections, worm infestation, and kidney and liver failure. Signs of a fish with dropsy include a large, swollen abdomen, bulging eyes, inflamed anus as well as pale gills. 

This symptom in itself isn’t contagious. However, when a fish has dropsy, it normally indicates that there is an underlying illness. Thus, you should put any fish with dropsy in quarantine immediately. Any type of fish can have dropsy.

Check this article to know more about on How to Treat and Prevent Deadly Dropsy In Fish?

Malawi Bloat

Malawi Bloat, also called Cichlid Bloat, is a condition that causes excessive bloating in fish. The disease got its name because of the Malawi African Cichlids that were infected by it. However, the exact cause of Malawi Bloat is still unknown. Experts debate whether a parasite or bacterium causes this disease. 

Signs of Malawi Bloat include abdominal swelling or bloatedness, red anus, breathing difficulties, and anorexia or appetite loss.


We know how critical taking care of your pet fish is to you. With the information above, you have enough to equip yourself with the right knowledge to use them in the future when needed. Aquarium fish diseases almost always come unexpectedly. Thus, the real key to this problem is prevention. Veterinarians are always there to help out if ever things go out of hand. Remember that if something goes wrong and you’re unsure of what to do, ask for a health professional’s advice.