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19 Common Freshwater Fish Illnesses with Symptoms and Treatments (Pictures Included)

Freshwater fishes are fishes that mostly live in freshwater or in areas where salinity is below 1.05%. 

They are hardy and easy to maintain. With proper care and suitable habitat, you can keep freshwater fish healthy and happy.

The primary causes of illnesses in fishes are stress, improper habitation, bad water quality, and bad diet. 

Therefore, it is easy for freshwater aquarists to keep their fish healthy and comfortable.

Additionally, early stress or illness detection can save your fishes from prolonged or advanced health complications.

Most aquarists encounter complex health issues in their fishes due to ignorance. Therefore, we came up with this article to help you understand the most common illnesses in freshwater fishes.

Keep your eyes open to learning more about these 19 common freshwater fish illnesses. Stay tuned.  

What Are Common Freshwater Fish Illnesses

Even though freshwater fishes are hardy, they occasionally get sick due to stress and impoverished habitation. Some illnesses are common among freshwater fishes but also treatable. Knowing how to identify an ailing fish is critical for the fish hobbyist to salvage them from dying.

Fish are sensitive, and slight environmental changes can trigger stress and illnesses. 

Multiple bacterial, fungal, and parasitic infections can affect freshwater fishes.

However, with proper care and quality water, you can keep your fish free from those infections.

Every aquarist should know how to maintain their type of fish to keep them healthy and stress-free.

It is, therefore, necessary to prevent and prepare yourself in case those diseases and infections hit your aquarium.

Here are some tips for you:

  • Set aside a small hospital tank where you can isolate and treat ailing fishes. It is cheap to treat only the sick fish instead of the entire aquarium.
  • Carbon absorbs medication from the water. Thus, always remove any activated carbon when treating your fishes.
  • Add aquatic salt to the freshwater aquarium to promote good health, reduce stress, and heal fish wounds.
  • Add multivitamins regularly in your aquarium to boost your fish’s immune system.
  • Add an aquarium filter in your tank and regularly change the water to maintain quality water and reduce stress from your fish.

Following the above-listed tips can help you to prevent and treat the most common infections in fishes successfully.

19 Common Freshwater Fish Illnesses

If you’re yet to experience sickness in your freshwater fish, it is imperative that you read about signs of a healthy fish so as to know when something is wrong with your fish.

Prevention as always is better and cheaper than cure. it is also the best way to keep your fish healthy. However, if any fish disease or infection hits your aquarium, it is critical to quickly spot the signs for successful treatment.

Therefore, in this section, we will list the most common illnesses in freshwater fishes.

Additionally, we will tell you the causes, symptoms, and how to treat those illnesses.

Ich (Ichtyophthirius)

Ich is a parasitic infection that is common among stressed fishes. It affects both salt and freshwater fishes.

Ich infection can be fatal if left untreated for a prolonged period. It is, therefore, necessary to immediately isolate and treat the infection to prevent advanced symptoms.


  • Rapid breathing, especially at the top of the tank
  • White spots spreading from head to tail
  • Rubbing against  objects due to  irritation
  • Clamped fins


PH variations in your aquarium and rapid increase in temperature are the primary causes of ich in fishes.


  • Increase the aquarium temperature to 860F during the treatment period.
  • Add aquarium salt and regularly change the water.
  • Use chemical treatment. But, only if the above methods don’t treat. Chemicals can cause other health issues and thus should be the last option.
watch to see some common freshwater fish diseases explained with pictures [source: KeepingFishSimple]

Hemorrhagic Septicaemia

Hemorrhagic septicemia is a dangerous infection that may affect your freshwater aquarium. It mainly affects fishes that have viral infections.


  • Open sores
  • Bulging eyes
  • Distended abdomen 
  • Hemorrhaging skin and internal organs
  • Red bruising-like eyes, skin, and gills


You can use antibiotics but only to increase the mortality rate as hemorrhagic septicemia has no cure.


Dropsy is a bacterial infection that is caused by a bacteria known as Aeromonas. The bacteria live in fish aquariums but only affect your fish when they are stressed.

It may affect fish’s kidneys, causing fluid retention.


  • Swollen abdomen
  • Protruding scales
  • Lethargy 
  • Staying at the bottom of the tank
  • Sunken eyes
  • Swollen and bulging eyes
  • Loss of appetite


  • Isolate the fish and keep the hospital tank bare except for the heater and filter.
  • Add 2.5 teaspoons of Epsom salt in every 10 gallons of water to create a salt bath. It helps to absorb excess fluid from the fish’s body.
  • Mix your fish’s food with 1% antibiotic such as tetracycline or chloromycetin for at least ten days.
  • Treat the aquarium water with Maracyn Two, but only if the above methods do not seem to work.

Poor water quality is often the cause of most diseases and illnesses in freshwater fish. especially if the water used in the aquarium is tab water. The high chlorine content in tab water can cause serious issues to freshwater fish.

To eradicate this, you should ensure that the water in your fish tank is properly treated to prevent diseases.

To achieve this, we recommend API Stress Coat Water Conditioner or Maracyn Two for water treatments. These products make tap Water Safe, replace your fishes’ protective coat damaged by handling or fish fighting.

For optimal result, use any of these products when adding or changing the water, just adding fish to tank, and when fish are injured.

Last update on 2022-12-29 / Affiliate links / Images from Amazon Product Advertising API

Fin Rot

Fin rot is a common infection among all fishes. It is caused by pseudomonas, Aeromonas, or Vibrio bacteria. 

It is challenging to cure fin rot, especially in advanced stages. It is also highly contagious and can quickly kill all fishes if left unattended. 

Luckily, it is easy to prevent fin rot by maintaining quality water, adequate temperature, proper feeding, and overcrowding.

Generally, fin rot mainly attacks stressed fishes.


  • Discolored fin edges
  • Frayed and uneven fin edges
  • Rotten fin and tail
  • Loss of appetite
  • Lethargy


  • Enhance your fish’s habitat
  • Eliminate all possible reasons stressing your fish
  • Clean the fish tank
  • Use broad-spectrum antibiotics such as erythromycin.

Piscine Tuberculosis

Piscine tuberculosis is a bacterial infection that affects fish, humans, birds, and reptiles. 

It is caused by a bacterial known as mycobacterium spp. It is a common environmental contaminant that does not require a host.

Thus, you should always wash your hands thoroughly after and before handling your aquarium.


Tuberculosis may not have any clinical signs. However, the mortality rate will be very low.


  • A non-symptomatic new fish in the aquarium
  • Infected accessories in the tank
  • Transfer from your hands to the tank 


  • There is no cure for tuberculosis in fish

Mouth Fungus

Mouth fungus is a bacterial infection that may affect freshwater and saltwater fish. The bacteria that cause mouth fungus is known as Flavobacterium and is always present in the aquarium.

However, it can also be brought about by new infected fish. Therefore, it is critical to isolate new fish before introducing them to your aquarium.

Even though Flavobacterium is always present in the aquarium, it remains dormant and only affects stressed or injured fish.


  • White or grey discharge in your fish’s mouth, face, or gills
  • Loss of appetite
  • red ulcers on fins and body
  • lethargy
  • clamping fins
  • pale body


  • Poor water condition
  • Stress
  • Improper diet


  • Isolate the fish
  • Clean the tank filter any active carbon
  • Treat the aquarium with Maracyn 
  • add aquarium salt in the tank

Swim Bladder

The swim bladder is also known as flip over. It affects the fish’s ability to swim due to gas-filled organs.


  • Swimming upside down at the top of the tank
  • Sinking at the bottom of the tank or floating at the top
  • Distended belly
  • Curved back
  • Loss of appetite


  • Rapid eating
  • Overeating
  • Constipation 
  • Parasites or bacterial infections 
  • Enlargement of other internal organs due to egg binding, kidney cyst


  • Stop feeding the fish for at least three days
  • Increase the temperature to 78 – 800F during treatment 
  • Feed frozen peeled peas on the fourth day
  • Add a small amount of aquarium salt to the hospital tank
  • Reduce water level 


Vibrio bacteria are the main culprit for Vibriosis diseases. It is a common disease among freshwater fishes and other marine animals. The fish can stay for approximately three days before exhibiting any clinical signs after the infection.


  • Anorexia
  • Abdominal distention 
  • Dermal hemorrhages
  • Anemia
  • Red spots appear around ventral and lateral areas
  • The liver, kidney, and spleen may swell or become congested
  • during the advanced stages, the fish may have Pale gills and fin rot
  • sudden death without clinical sign


V. anguillarum bacteria is the primary cause of vibriosis. The bacteria can stay active in the water for more than 50 months.

Stress factors can trigger the bacteria causing the fishes to suffer from fibrosis. These stress factors include:

  • Overcrowding 
  • Poor nutrition
  • Bad water quality
  • High water temperature
  • Organic pollution in the aquarium 
  • Improper handling


  • Quarantine all sick fishes
  • Eliminate all stress factors
  • Oxytetracycline for ten days
  • Sulphamerazine 
  • Furazolidone 

Last update on 2022-12-30 / Affiliate links / Images from Amazon Product Advertising API


Pseudomonas is a common infection amongst freshwater fishes and other marines. 

It is caused by a bacteria known as pseudomonas spp. The bacteria have several species and are always present in the aquatic environment.

They do not cause any health issues unless triggered by other stress factors.


  • Ulceration
  • Fin erosion 
  • Lethargy
  • Sudden death
  • Loss of appetite


Pseudomonas spp. It can inactively lie in an aquarium without any effect. However, stress triggers the bacteria causing it to thrive and lead to pseudomoniasis infection.


  • Eliminate all causes of stress
  • Consult a vet to provide an effective antibiotic
  • Pseudomonas spp. is resistant to most antibacterial medication. Thus, over-the-counter medicines may not work.


Saprolegniasis is an infection caused by fungi in the genus Saprolegnia. It affects freshwater fish and fish eggs.


  • Cotton-like growths on gills and skin
  • Sunken eyes
  • Discoloration of skin
  • Cotton-like growth in the muscle tissue in advanced stages


  • The extremely low temperature usually below 590F during winter 
  • Overcrowding 
  • Poor water quality


  • Harvest the infected fishes 
  • Observe proper habitat sanitation
  • Clean the tank to prevent further contamination


Lymphocystis is a chronic viral disease that affects freshwater and marine fishes. It is caused by a virus known as lymphocyistivirus


  • Slight or moderate wart-like growth on fins, gills, and skin
  • Popeyes


  • Contagiousness from one fish to another
  • External trauma, overcrowding, and other stressors can trigger the virus.


  • Isolate the sick fishes
  • Provide a suitable habitat
  • Offer asymptomatic treatment

Skin Gill Flukes

Skin Gill Flukes in koi fish freshwater fish illnesses
Skin Gill Flukes in koi fish

Skin gill flukes are tiny worm-like parasites. They are microscopic and thus difficult to see.


  • Small holes that cause a secondary infection
  • Listless
  • Difficulty in breathing
  • Discolored gill filaments
  • Swollen gills 
  • Laying at the bottom


  • Monogram trematodes
  • Inadequate water quality and other stress factors can trigger the infection.


  • Clean the tank
  • Observe Proper diet and habitation

Anchor worm

They are freshwater copepods that attach to the freshwater fish’s skin and gills. 

The female anchor worm only attaches itself to the male fish and vice versa. The anchor worm can cause a secondary infection known as Lernaeosis parasitic disease. 


  • Short white worms behind scales
  • Baleen appearance on the fish mouth
  • Hemorrhage patches 


  • Transfer from one fish to another
  • Contaminated live plants


  • Consult a vet as the fish requires to be sedated to remove those anchor worms

Tetrahymena or Guppy Disease 

A baby fish with a Tetrahymena Or Guppy Disease
A baby fish with a Tetrahymena Or Guppy Disease 

Tetrahymena is common amongst freshwater and marine fishes. It resembles ich disease and requires microscopic analysis to identify it.

It deeply penetrates through the fish’s skin, causing hemorrhagic regions.


  • Whitish spots on the body
  • Hemorrhagic patches
  • Heavy breathing 
  • Lethargy 
  • Muscle swelling 


Ciliate protozoans in the genus Tetrahymena Corliss and Tetrahymena pyriformis are the primary cause of guppy disease. 

The tetrahymena parasites can stay in the aquarium without causing any infections. However, stress, deficient nutrients, bad habitation, and any other factor that may cause low immunity triggers the parasite causing guppy disease.


Unfortunately, there is no proven treatment for guppy disease. However, niclosamide, albendazole, and chloroquine may offer a certain degree of relief.

Lateral Line Disease 

It is also known as a “hole in the head.” It is caused by Hexamita protozoa and affects most marine aquatic including freshwater fishes.


  • White stringy feces
  • Cavities in the temples
  • Lateral line 


  • Poor water condition 


  • Metronidazole (Flagyl)

Fish Lice 

They are crustaceans in the genus Argulus. They appear with a broad flat shell and a set of four legs. 

The lice attach themselves to your fish’s body, and you can quickly identify them following a close observation. 

Fish lice are dangerous and can cause secondary infections if you do not eliminate them ASAP.


  • Erratic swimming 
  • Rubbing against objects


  • Introducing infected fish in the aquarium


  • Dimilin 
  • Physical removal using tweezers

Oodinium (Velvet, Gold Dust Disease)

It is a dinoflagellate that comes with several species and affects the fish’s skin and gills.


  • A dusting with a yellow to rust color on the fish’s body
  • Heavy mucus secretion 
  • Clamped fins 
  • Labored breathing 


  • Bad water quality
  • Bad nutrients
  • Poor habitation 


  • Acriflavine 
  • Copper sulfate 
  • Carbon water filter
  • Temperature rise to 860F


It is also a protozoan parasite infection that exhibits ich-like symptoms. It is common in all fishes, including freshwater fish.


  • Clamped fins
  • Heavy mucus secretion 
  • Scratching against objects
  • Gasping for the air at the surface
  • Labored breathing 


  • Poor water quality


  • Formaldehyde
  • Methylene blue
  • Acriflavine 

Nematode Worms

Anisakis Nematode parasite all over the liver of a fish
Anisakis Nematode parasite all over the liver of a fish

It is also known as roundworms. They either have direct or indirect life cycles. Direct lifecycles pass from one fish to the other. 

On the other hand, the indirect requires a secondary host such as invertebrates or other fishes.


  • Bloating 
  • Body hemorrhaging 
  • Worm protruding in the fish’s anus
  • White  feces


  • Introducing infected fishes in the aquarium
  • Feeding infected food


  • Levamisole 
  • Metronidazole 
  • Praziquantel 
  • Nitrofurazone and erythromycin antibiotics are also effective in preventing secondary infections.

Wrapping Up

The above listing of the diseases and infections can be worrying for any aquarist, especially beginners. However, a keen observant can also tell that most of these diseases have a common cause.

The most common causes of diseases and infections in fishes are bad water, quality, bad nutrients, inadequate habitation, and other stress factors.

Therefore, with proper care, you can easily keep your freshwater fishes happy and free from any bacteria, viral, fungal, or protozoan infection. 

Remember that prevention is better than cure. 

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