20 Most Common Diseases of Freshwater Aquarium Fish

Discover the 20 most common diseases that afflict freshwater aquarium fish. You’ll learn about their causes, symptoms, and treatments. Armed with this knowledge, you can ensure your aquatic companions live healthier, happier lives.

freshwater fish diseases

This page may contain affiliate links, which will earn us a commission. As an Amazon Associate we earn from qualifying purchases.

Ich (White spot disease)

Ich, also known as White Spot Disease, is one of the most common diseases your freshwater aquarium fish could get. It’s named after its notable symptom: white spots that form on the fish’s body, fins, and gills.


  • Ich manifests as small, white, salt-like spots on the fish.
  • Infected fish tend to scrape their bodies against rocks and decorations.
  • They may also exhibit abnormal behaviors such as lethargic movements or rapid breathing.


The cause of Ich is a parasitic protozoan called Ichthyophthirius multifiliis. This parasite spreads rapidly in the tank through spores and can infect other fish quickly if not treated promptly.


For treatment, you should first raise the water temperature by 2°F (approximately 1°C) each day until reaching 86°F (30°C), as this temperature can aid in killing the Ich parasite.

Another effective treatment method is a formalin-malachite green based medication, specifically designed to combat Ich. Be sure to follow the dosing instructions carefully.

During and post-treatment, remember to change at least 25% of the tank water daily. This helps to remove spores and keeps your aquarium environment clean and conducive to fish health.

Fin and Tail Rot

Fin and Tail Rot is a common disease that plagues your lovely aquarium inhabitants.


The red flags of this disturbing affliction are the fraying or disintegration of your fish’s fins and/or tail, which may also display a white edge to the affected areas. Additionally, your fish may display lethargic behavior, repeated rubbing on objects, and may refuse food.


This disease is primarily brought on by bacterial infections, most commonly Pseudomonas and Flexibacter. Stressful conditions, such as poor water quality, overcrowding, or aggressive tank mates, can make your fish more susceptible to these bacteria.


Fortunately, treatment is possible with a specific approach in time. First, improve the water conditions promptly as bad hygiene often is the major contributor.

Follow this with antibacterial medication available at pet stores, such as antibiotics marketed for fish. Keep in mind, early detection and treatment are vital for the survival and health of your fish.

Swim Bladder Disease

Swim Bladder Disease is a common issue among freshwater aquarium fish, particularly affecting the goldfish, guppies, and bettas. This condition, resulting from an inflamed swim bladder, causes your fish to lose its balance in the water.


  • Swimming Upside Down: Fish affected by Swim Bladder Disease may swim upside-down or lie flat at the bottom of the tank.
  • Peculiar Swimming Patterns: You may notice abnormal swimming patterns such as floaty, heavy-bottomed, or tilted swimming.


  • Overeating and Constipation: Overeating or high-fiber diet could cause constipation, leading to Swim Bladder Disease.
  • Physical Trauma: A sudden shock or physical injury can also cause inflammation in the swim bladder.


  • Dietary Changes: The easiest treatment is to change the fish diet. Feeding them with peas can be highly beneficial.
  • Medication: Use over the counter swim bladder treatment can also be considered.

Early detection of the disease is critical for successful treatment. Maintain a regular check on fish behavior and symptoms for keeping them happy and healthy.


If your fish is unusually round or bloated, it could be suffering from Dropsy. This condition, often indicative of serious internal problems, may cause the fish’s scales to stand noticeably on end.


  • Unexplained bloating or puffiness
  • Scales protruding unnaturally from the body
  • Abdominal swelling and discomfort

Confirmed diagnosis can be difficult as the disease manifests differently in various species. It’s primarily an internal malfunction, usually related to kidney failure. When the kidneys cannot evacuate fluids properly, it may lead to water retention and the subsequent bloating.


  • Bacterial Infection: This is the most common cause. Bacteria infiltrate the fish’s system, attacking the organs and compromising functionality.
  • Poor water quality: Overpopulation, inadequate filtration, and lack of regular water changes could contribute to your fish contracting Dropsy.


  • Isolate the affected fish to prevent the disease from spreading to others.
  • Use antibiotics carefully as the disease is bacterial. Typically, fish-specific antibiotics such as Kanamycin or Tetracycline work best.
  • Improve water quality by ensuring regular changes and ideal water parameters for your specific fish species.

Remember, prevention is the best cure for all fish diseases. Maintain good hygiene and water quality in your tank and feed your fish a balanced diet to ensure a stress-free ecosystem.

Velvet Disease

Velvet Disease is a common ailment among freshwater aquarium fish which is caused by a parasite called Oodinium pilularis. This parasite manifests as a rust-colored, dusty layer on the fish’s skin, hence its name.


  • Fish’s skin appears rusty or yellowish
  • Excessive slimy skin
  • Rubbing against objects due to itchiness
  • Rapid gill movement and respiratory distress

The parasite thrives in poor water conditions, stressing the fish and making them more susceptible. Therefore, maintaining a clean tank is essential to prevent the disease.


Fortunately, Velvet is treatable, usually with copper-based medications. Some steps are:

  • Isolate affected fish to halt the spread of this contagion.
  • Treat the entire tank with a copper-based medication.
  • Follow the medication instructions carefully, overuse could harm your fish.

Remember, prevention is better than cure. Regular monitoring and maintaining the pristine condition of the tank can help your fish keep their healthy glow.

Columnaris (Cotton Mouth)

Columnaris is a bacterial infection, notorious among freshwater aquarium fish. Often mistaken for a fungal infection due to its fuzzy, cotton-like growths, it poses a significant threat to your fish’s health.


  • white or grayish layer appears on the mouth, skin, and gills of the fish, giving it a cottony look hence its nickname ‘Cotton Mouth’.
  • Your fish may exhibit loss of appetite, difficulty breathing, and lethargy.


  • This disease is primarily caused by Flavobacterium columnare, a bacteria that thrives in higher temperatures and poor water quality.
  • Overcrowding in the aquarium, stress and physical damage can also provide ideal conditions for this bacteria to multiply and cause infection.


  • Columnaris can be treated with antibacterial medications specifically designed for fish, such as Kanamycin or Erythromycin.
  • Additionally, improving the aquarium’s hygiene and reducing stress for the fish can drastically improve recovery rates.

Always remember, early detection and prompt treatment of Columnaris can significantly increase your fish’s chances of survival. Control the spread of this disease by quarantining infected fish and maintaining optimal water conditions in your aquarium.

Gill Flakes

Gill flukes are common parasites that cause serious health problems in freshwater fish. These are small, flat worms that attach themselves to your fish’s gills, causing irritation and damage.


  • Frequent rubbing or scratching against objects
  • Rapid gill movement
  • Difficulty in breathing
  • Pale or discolored gills


Gill flukes are caused by two types of parasitic flatworms: Dactylogyrus and Gyrodactylus. Dactylogyrus are egg-layers and their life cycle takes place entirely on the host fish. Gyrodactylus are live-bearers and can survive for some time without a host.


There are some effective treatments for gill flukes. One of the most effective is Praziquantel, a broad-spectrum anti-parasitic medication. Alternatively, you can also use Formalin or Copper-based treatments. Please remember to treat the whole tank as these parasites are highly contagious and can spread rapidly.

Always consult with a professional if unsure.

Remember, maintaining a clean and stable environment in your aquarium is crucial to preventing these types of freshwater fish diseases.

Anchor Worm

Anchor Worm is a common disease of freshwater fish, mainly brought in by new fish from untrustworthy sources. In essence, it’s a parasite infection that creates numerous health issues for fish.


Look out for fish rubbing themselves on aquarium décor or the tank itself. They might show signs of discoloration, lethargy, or spicy red ulcers where the worm grips the fish. Often, you will see the worm itself anchored onto the fish.


This illness mainly infects a tank if you introduce a new inhabitant that’s already infested with the parasites. Anchor Worms are not common in well-maintained aquariums; they often strike where fish are stressed, overcrowded, or in poor water conditions.


There’s no denying that Anchor Worm is a severe disease, but there are treatments. You can manually remove visible worms with tweezers – be gentle to not harm your fish’s scales.

After that, manage this ailment using anti-parasitic fish medication available at pet stores. Remember to always quarantine new fish before introducing them to your main tank to prevent such invasive parasites.

The key to dealing with ‘Anchor Worm’ lies in early detection, prompt action, and preventive measures. The health of your aquatic friends often mirrors your care and attention towards their habitat. Be vigilant and stay proactive.

Fish Tuberculosis (Mycobacteriosis)

Fish Tuberculosis, also known as Mycobacteriosis, is a bacterial infection prevalent in freshwater aquarium fish. This chronic disease poses a significant threat as it’s highly infectious and can be deadly.


  • Unexplained weight loss and loss of appetite
  • Frayed and decolorized fins
  • Abnormal behavior such as isolation or laggard swim pattern
  • Presence of granulomas or skin lesions


The Mycobacterium marinum bacterium is the root cause of Fish Tuberculosis. This bacterium thrives in both saltwater and freshwater environments but more frequently affects fish in aquariums due to the closed nature. It can spread through contaminated water, shared equipment, or direct contact with infected fish.


Fish Tuberculosis is a challenging disease to treat due to its resistance to many antibiotics. The most effective course of action includes:

  • Antibiotics: Administer kanamycin and vitamin B-6 combination for a prolonged period.
  • Tank Maintenance: Regularly clean and disinfect the aquarium, ensuring a healthy environment.
  • Preventive Measures: Quarantine new fish before introducing them to the main tank.

Remember, this disease is zoonotic, meaning it can be transferred to humans. So, always wear waterproof gloves when handling infected fish or when cleaning the aquarium.


Lymphocystis is a common viral infection that affects freshwater fish. It manifests as small, pale or white nodules protruding from the fish’s skin, fins, or mouth.


Infected fish develop cauliflower-like growth on their skin and fins. These growths can vary in size, from barely visible to large pinhead-sized bulges.


This is a viral disease and is transmitted when healthy fish come into contact with affected ones. It’s more prevalent in overcrowded and stressful tank environments.


There’s no appropriate medication for Lymphocystis. However, enhancing the fish’s environment and diet can boost their immune system.

This assists them to overcome the infection on their own. Regular water changes, maintaining an ideal temperature and pH, and providing a balanced diet are crucial steps towards recovery.

Cotton Wool Disease (Saprolegnia)

Cotton Wool Disease, or Saprolegnia, is one of the most common diseases that can affect freshwater aquarium fish. This fungal infection is named for its distinctive appearance, often resembling tufts or patches of cotton on the fish’s body.


  • Fluffy white or grey growths on the skin, fins or gills
  • Gasping for breath if the gills are affected
  • Slow movement or listlessness
  • Discoloration or redness


  • Poor water quality through infrequent tank cleaning or overfeeding
  • Injury or damage to the fish’s scales
  • Stress, often caused by transportation, poor diet, or overcrowded tank conditions


Thankfully, Cotton Wool Disease is treatable. Begin by improving the water quality in your tank; regular cleaning and maintenance are crucial. Carefully using antifungal medications available at most pet stores can also help. However, always consult with a vet or an aquarium specialist before starting any treatment.

Remember, healthy tank conditions can prevent many diseases, including Saprolegnia. Good hygiene and regular monitoring of your aquarium can make a significant difference in your aquarium fish’s health.

Hexamita (Hole-in-the-head disease)

Hexamita, often referred to as ‘Hole-in-the-head disease,’ is a common concern for freshwater aquarium fish. This condition, caused by a flagellate protozoan, results in tell-tale symptoms that are hard to ignore.


  • Freshwater fish affected with Hexamita may display small pits or ‘holes’ around the head region.
  • A subsequent loss of appetite and discoloration of the skin are also observed.
  • In severe cases, fish may also exhibit abnormal swimming behavior.


  • Hexamita is primarily caused as a result of poor water quality.
  • Overcrowding the aquarium and a poor diet can trigger the condition.


Fortunately, Hexamita is treatable. It’s important to take actions swiftly as soon as you recognize the symptoms.

  • The first step towards the recovery of your fish is to improve the conditions of the aquarium by regular water changes and reducing overcrowding.
  • Then, for the actual treatment, over-the-counter medications containing metronidazole are effective. A swimming bath with the recommended dosage can do wonders.
  • For a nutrition boost, increase vitamins in the diet, particularly Vitamin C.

It’s often simple to prevent diseases by providing a favorable environment for your fish. Proper water conditions, a balanced diet, and ample space are key to a happy and healthy aquarium.


Pop-eye, often observed in freshwater aquarium fish, is an easily noticeable disease due to the swelling of one or both eyes. This condition can be quite alarming to fish keepers.


  • Noticeable protrusion of one or both eyes
  • Enlarged eye size that possibly induces cloudy or glassy-eye outlook

The above presentations signal you to the possible advent of the dreaded Pop-eye.


This disease is essentially caused by either:

  • An internal infection
  • An injury directly done on the eye

Poor water quality and unfortunate incidences like aggressive fights among fish may lead to these causes.


The primary course of treatment for Pop-eye involves:

  • Increasing water cleanliness and quality
  • May include the administration of broad-spectrum antibiotics

Nitrate levels should be kept less than 20 ppm and nitrite levels should be kept less than 1 ppm. Regular water changes, with dechlorinated water, are beneficial for recovery.

Mouth Rot

Mouth rot, also known as ‘Cotton Mouth’ or ‘Columnaris’, is a common, contagious bacterial infection that can affect both marine and freshwater fish. This disease is often caused by poor water quality and overcrowded conditions, meaning that close attention to tank cleanliness and population rate is key to avoiding it.


  • Fish have a white, cotton-like growth around the mouth.
  • The affected fish may show signs of poor appetite or not eat at all.
  • They may experience breathing difficulties due to inflamed gills.


  • The main cause of mouth rot is a bacterium named Flavobacterium Columnare.
  • Stress can make fish more vulnerable to mouth rot. Stressors may include overcrowding, inadequate diet or poor water quality.


  • Incorporate regular water changes to improve quality.
  • Fish can be treated with antibiotics, like tetracycline or erythromycin.
  • Apply a disinfectant to the fish tank to prevent further spread of the bacteria.

Following these steps will ensure the health of your aquarium and prevent the outbreak of mouth rot. Remember to watch the behavior and appearance of your fish.

Changes can often indicate the onset of disease. Maintain a proper feeding schedule and clean your tank regularly to significantly reduce the risk of mouth rot transmission.

Fish Lice

Fish Lice, scientifically known as Argulus, is a common parasitic disease in freshwater aquarium fish. This troublemaker clings onto the host fish’s body, causing severe discomfort and, in worse case scenarios, death.


  • Fish infested with Fish Lice are often seen scratching against objects in the tank.
  • You’ll spot tiny, flat, disc-like organisms glued onto the fish’s body.
  • Signs of stress are evident, with the diseased fish often displaying rapid gill movement.


  • Introduction of new fish carrying the parasite.
  • Contaminated plants or decor introduced into the aquarium.
  • Poor sanitation and hygiene of the aquarium environment.


If you think your tank has an outbreak of fish lice, you need to act promptly.

  • Start by physically removing visible lice with a pair of fine tweezers.
  • Second, a commercial anti-parasitic treatment, available in most aquarium specialty stores, should be applied.
  • Improving the overall cleanliness and hygiene of your tank can help prevent future breakouts.

Remember, Fish Lice not only affect the health of your aquatic pals, but can also disrupt the peace in your underwater community. Be proactive and vigilant to ensure the comfort and longevity of your fish.

Neon Tetra Disease

Recognizing the Neon Tetra Disease is crucial due to the high rate at which it spreads. Parasites cause this disease, mainly Pleistophora hyphessobryconis, affecting the aquarium fishes, particularly the Neon Tetras. Once infected, a cure is improbable.


  • The first symptom to notice is the color fading of neon tetras. Their vibrant blue stripe fades to a pale color.
  • The infected fish could also exhibit impaired swimming, restlessness, and a loss in appetite.
  • As the disease advances, cysts begin forming, leading to asymmetrical deformation.


  • The primary cause of this disease is the Pleistophora hyphessobryconis parasite. The parasite enters the fish’s body when it eats the dead remains of another infected fish.
  • Poor water quality – keeping the water unclean accelerates the growth of parasites.


Unfortunately, there is no reliable cure once a fish is infected with neon tetra disease. It is better to remove and euthanize the affected fish immediately to prevent the spread of the disease. Regular water changes, removal of leftover food, and ensuring a healthy diet for your fish are crucial preventive measures.

Cloudy Eye

Cloudy Eye is a common disease in freshwater aquarium fish. It’s a symptom of an underlying issue rather than a disease in itself.


When a fish has Cloudy Eye, you’ll notice a gray or white haze over the eyes. Its eyes may become swollen and look like they are covered with a thin film or layer. You may also notice a decrease in your fish’s appetite and general sluggish behavior. In severe cases, the fish might lose vision completely.


Cloudy Eye generally occurs due to poor water quality. It can also be caused by bacterial infections or physical injuries. Various factors such as elevated nitrate levels, drastic pH shifts, chlorine present in the water, and inadequate diet can contribute to this problem.


Maintaining water quality is essential in preventing and treating Cloudy Eye. Start by replacing 25% of the tank water and cleaning the fish tank thoroughly.

If the situation does not improve within a week, using an antibacterial treatment (like tetracycline or erythromycin) is recommended. Always remember to observe your fish for any changes in their behavior or appearance.

Ichthyobodo (Costia)

One of the most prevalent diseases you may face as an aquarium owner is Ichthyobodo, commonly known as Costia. These microscopic parasites can lead to serious health issues in your fish if left unnoticed and untreated.


  • Your fish may become lethargic and lose appetite.
  • They may also exhibit heavy breathing and extra mucus production.
  • Also, watch out for a grayish-white skin sheen appearing on the fish’s body.


  • The cause of Costia is a flagellate parasite called Ichthyobodo necator.
  • High levels of organic matter in water, and poor water quality can increase the chances of infection.
  • Overcrowded tanks and stress can also facilitate Costia outbreak.


  • Formaldehyde and malachite green are used in treating Costia.
  • Also, improving water quality is a must.
  • To avoid a recurrence, take prompt action when the initial signs appear.

The sheer microscopic size of Ichthyobodo parasites makes them hard to detect, but they can wreak havoc in your aquarium. Therefore, maintaining a clean tank is of utmost importance to prevent this disease.

It’s not just about the aesthetics, but also about the health of your finned friends. Stay vigilant and act swiftly if you notice any signs of infection.

Camallanus Worm

The Camallanus Worm is a common but serious parasite infestation in freshwater aquarium fish. This disease manifests primarily in livebearer fish, and is quite easily recognizable.


  • Your fish may exhibit a reduced appetite.
  • They might seem to struggle with their balance or display an irregular swimming pattern.
  • The most distinctive sign is the appearance of red worm-like protrusions from the fish’s anus.


  • The main cause is the introduction of an infected new fish to the tank.
  • Also, feeding your fish live food that is infested can spread the worm.


  • The most effective treatment is Levamisole hydrochloride.
  • Firstly, remove visible worms with tweezers. Then, treat the whole tank for several weeks to eliminate all lifecycle stages of the worm.
  • It’s important to treat even the asymptomatic fish as they might be carriers.

Remember, Camallanus Worm is highly contagious and can quickly infest your entire tank if not adequately moderated. Preventive measures like quarantining new fish and feeding safe food can help keep your aquarium healthy. Regular water changes and maintenance can also control the spread of this parasite.


Scoliosis, a common affliction found in freshwater aquarium fish, is characterized by a noticeable curvature of the fish’s spine. This condition isn’t a disease in itself but a symptom of a variety of potential underlying problems. This can make it harder to specify the causes and treatment plan.


  • A visibly bent or curved spine.
  • Difficulty swimming or abnormal swimming patterns.
  • The Fish might show decreased appetite or weight loss.

The causes of Scoliosis in fish are diverse and not easily pinned down. However, some of the commonly identified causes are as follows:


  • Genetic predisposition or birth defects.
  • Nutrient deficiency, especially Vitamin C.
  • Certain bacterial or viral infections.
  • Metabolic bone disease and aging.

Treatment for Scoliosis in fish can be challenging as it depends on the underlying cause. The most effective approach focuses on prevention and proper care.


  • Ensuring a balanced diet rich in essential nutrients.
  • Regular water changes and maintaining optimal water conditions.
  • Isolate affected fish to prevent spreading potential infectious agents.

Remember, Scoliosis isn’t always a sign of neglect or lack of care. However, prompt attention can help minimize impact and prevent it from affecting other fish in your aquarium.


To keep your freshwater aquarium fish healthy and vibrant, it is essential to stay vigilant for these common symptoms and diseases. A proper diagnosis and prompt treatment can ensure the well-being of your aquatic friends. Please feel free to leave a comment and share your experiences in dealing with these common diseases.

Leave a Comment

Your email address will not be published. Required fields are marked *