Just as you recognize the early signs of a cold and know when to visit a doctor, identifying illness in aquarium fish is crucial to their well-being. Unlike humans, fish cannot communicate their discomfort, making it challenging to detect health issues before they become serious. Many fish diseases develop subtly, but without timely intervention, they can progress and even become fatal. Early diagnosis and treatment are essential for recovery.
For fish in a controlled environment, every small change matters. Even a slight fluctuation in water temperature can cause stress, weakening their immune system and increasing susceptibility to disease.
This guide will cover the most common aquarium fish diseases, helping you recognize their symptoms, understand their causes, and learn the best treatment methods to keep your fish healthy.

Common Fish Diseases:

Below is a list of some of the most frequently encountered fish diseases:

1. Dropsy

                           

Dropsy, also referred to as edema, is not a disease itself but a symptom of an underlying health issue, typically a serious internal infection. It is most recognizable by a swollen, bloated abdomen and protruding scales, giving the fish a “pinecone” appearance.

Symptoms of Dropsy:

• Swollen or bloated belly

• Scales sticking out from the body (pineconing)

• Lethargy and loss of appetite

• Difficulty swimming or staying upright

Causes & Treatment:
Dropsy is commonly caused by bacterial infections, often due to poor water conditions or a weakened immune system. It can also result from parasites, organ failure, or viral infections.

Treatment involves:

• Isolating the affected fish immediately

• Improving water quality

• Administering antibacterial medications such as amoxicillin or kanamycin

• Adding Epsom salt to reduce internal swelling

• Using Metronidazole if a parasitic cause is suspected

Unfortunately, dropsy is often fatal if not caught early. Quick action and proper care are essential to give your fish the best chance of recovery.

Find out more about dropsy in fish .

2. Fin Rot

                           

Fin rot, sometimes referred to as rot fin, is a common and potentially serious condition affecting aquarium fish. It is characterized by the gradual fraying, discoloration, or deterioration of a fish’s fins or tail. In more advanced stages, the fin tissue may become inflamed at the base or even rot away completely.

Symptoms of Fin Rot:

• Fins or tail turning a white, brown, or black color

• Frayed or ragged fin edges

• Inflammation or redness at the base of the fins

• Progressive deterioration of fins or tail

• Lethargy and reduced activity

Causes & How to Treat Fin Rot:

Fin rot is most often the result of poor water quality, which stresses the fish and compromises their immune system. It can also be triggered or worsened by aggressive tank mates who nip fins or by overcrowded conditions.

How to cure fin rot:

• Begin by improving water quality: perform a partial water change and test water parameters.

• Isolate the infected fish, especially if aggression from others is suspected.

• Use antibacterial treatments such as amoxicillin, or fin rot-specific medications .

• If fungus is also present, consider antifungal treatments.

• Boost your fish’s diet with high-quality, vitamin-rich foods to promote healing.

Fin rot treatment is most effective when caught early. Regular monitoring of water conditions and your fish’s behavior is key to prevention.

Find out more about fin rot(fish fungus).

3. Ammonia Poisoning

Cause:

Occurs when toxic ammonia levels rise due to decomposing organic matter, overfeeding, or inadequate filtration.​

Symptoms:

• Gasping at the surface

• Red or inflamed gills

• Lethargy

• Loss of appetite

• Clamped fins

• Red streaks on the body​

Treatment:

• Immediate partial water changes (25–50%)

• Use of ammonia-neutralizing products

• Enhance filtration

• Reduce feeding

• Ensure the tank is properly cycled before adding fish​

4. Anchor Worms (Lernaea)

Cause:

Parasitic crustaceans that embed into fish tissue, often introduced via new fish or plants.​

Symptoms:

• Visible worm-like protrusions

• Red, inflamed skin

• Ulcers at attachment sites

• Rubbing against objects​

Treatment:

• Manual removal with sterilized tweezers

• Apply antiseptic to wounds

• Use of antiparasitic medications like diflubenzuron (Dimilin)

• Quarantine affected fish

5. Bacterial Infection (Aeromonas)

Cause:

Opportunistic bacteria infecting fish under stress or with injuries.​

Symptoms:

• Swollen eyes

• Abdominal bloating (dropsy)

• Skin ulcers

• Hemorrhagic lesions

• Ragged fins​

Treatment:

• Isolate affected fish

• Administer broad-spectrum antibiotics

• Improve water quality

• Address underlying stressors

6. Brooklynellosis (Anemonefish Disease)

Cause:

Protozoan parasite Brooklynella hostilis, commonly affecting clownfish.​

Symptoms:

• Excessive mucus production

• Sloughing skin

• Respiratory distress

• Lethargy​

Treatment:

• Formalin baths (45–60 minutes)

• Transfer to a clean quarantine tank

• Administer metronidazole

7. Columnaris (Mouth Fungus)

Cause:

Bacterial infection by Flavobacterium columnare, often mistaken for fungal disease.​

Symptoms:

• Cotton-like growths on mouth, fins, or gills

• Ulcers

• Frayed fins

• Lethargy​

Treatment:

• Use Antibiotics 

• Improve water quality

• Add aquarium salt to reduce stress

Find out more about Columnaris Disease in Fish

8. Flukes

Cause:

Parasitic flatworms affecting skin or gills.​

Symptoms:

• Scratching against objects

• Excess mucus production

• Labored breathing

• Clamped fins​

Treatment:

• Use antiparasitic medications 

• Quarantine affected fish

• Improve water quality

9. Gold Dust Disease (Velvet)

Cause:

Protozoan parasites (Piscinoodinium spp.) causing a dusty appearance.​

Symptoms:

• Fine yellow or gold dust on skin

• Clamped fins

• Rapid gill movement

• Lethargy​

Treatment:

• Raise water temperature gradually

• Dim aquarium lights

• Administer copper-based medications

• Quarantine affected fish

10. Hexamitiasis (Spironucleosis)

Cause:

Hexamitiasis is caused by protozoan parasites, primarily Hexamita and Spironucleus species. These parasites thrive in the intestines of fish, especially under stress conditions such as overcrowding, poor water quality, or after shipping.​

Symptoms:

• Weight loss despite normal feeding

• Loss of appetite

• White, stringy feces

• Lethargy

• Darkened coloration

• Hollowed abdomen​

Treatment:

• Medicated Feed: Administer metronidazole at 50 mg/kg of fish body weight for 5 consecutive days. If the fish is still eating, this is the preferred method.

• Bath Treatment: If the fish has stopped eating, use a bath treatment with metronidazole at 5 mg/L every other day until the parasite count decreases to zero. 

Prevention:

• Maintain optimal water quality.

• Avoid overstocking and reduce stressors.

• Quarantine new fish before introducing them to the main tank.​

11. Hole-in-the-Head Disease (HITH)

Cause:

Often associated with Hexamita infections, HITH is characterized by lesions on the head and lateral line. Poor water quality, nutritional deficiencies, and stress can exacerbate the condition.​

Symptoms:

• Lesions or pits on the head and along the lateral line

• Erosion of skin and muscle tissue

• Loss of appetite

• Lethargy​

Treatment:

• Medicated Feed: Use metronidazole-infused food at 50 mg/kg of fish body weight for 5 days.

• Water Treatment: If the fish isn't eating, administer metronidazole at 5 mg/L in a bath treatment every other day.

• Ensure pristine water conditions and provide a balanced diet rich in vitamins.​

Prevention:

• Regular water changes and filtration maintenance.

• Provide a varied and nutrient-rich diet.

• Minimize stress by avoiding sudden changes in the environment.

12. Ich (White Spot Disease)

Cause:

Caused by the protozoan parasite Ichthyophthirius multifiliis, commonly known as "Ich." The parasite attaches to the fish's skin and gills, forming cysts.​

Symptoms:

• Small white spots on the skin, gills, and fins

• Rubbing or scratching against objects (flashing)

• Rapid breathing

• Lethargy

• Loss of appetite​

Treatment:

• Temperature Increase: Gradually raise the water temperature to 78-80°F (25-27°C) to speed up the parasite's life cycle.

• Medications: Use copper sulfate or formalin-based treatments as per manufacturer's instructions.

• Salt Treatment: Add non-iodized aquarium salt at 1 tablespoon per 5 gallons of water. ​

Prevention:

• Quarantine new fish for at least 2 weeks.

• Maintain stable water parameters.

• Avoid sudden temperature fluctuations.

Find out more about fish ich (White Spot Disease).

13.Lymphocystis

Cause:

A viral infection caused by the Lymphocystivirus, affecting both freshwater and marine fish. The virus causes cells to enlarge, forming visible nodules.​

Symptoms:

• Cauliflower-like growths on the skin, fins, or gills

• Lesions may be white or pinkish

• Affected areas may become ulcerated over time​

Treatment:

• No specific antiviral treatment exists.

• Maintain excellent water quality to support the fish's immune system.

• Isolate affected fish to prevent spread.

• In severe cases, surgical removal of lesions may be considered. ​

Prevention:

• Quarantine new fish before introducing them to the main tank.

• Avoid stressors and maintain optimal tank conditions.

14.Marine White Spot Disease (Marine Ich)

Cause:

Caused by the protozoan parasite Cryptocaryon irritans, affecting marine fish. Similar in appearance to freshwater Ich but requires different treatment.​

Symptoms:

• White spots on the skin, fins, and gills

• Scratching against objects

• Rapid breathing

• Lethargy

• Loss of appetite​

Treatment:

• Copper-Based Medications: Use as per manufacturer's instructions, ensuring proper dosage.

• Hyposalinity: Gradually reduce salinity to 11-13 ppt under controlled conditions.

• UV Sterilization: Helps reduce free-swimming parasite stages.

Prevention:

• Quarantine new marine fish for 4-6 weeks.

• Maintain stable salinity and temperature levels.

• Avoid introducing wild-caught specimens without proper quarantine.

15.Parasite Infestation

Cause:

Parasite infestations in aquarium fish are common, especially in environments where fish are stressed due to overcrowding, poor water quality, or inadequate nutrition. These parasites can be external (ectoparasites) like anchor worms, epistylis, and flatworms, or internal (endoparasites) such as roundworms.​

Symptoms:

• Visible worms or threads protruding from the body

• Inflamed or reddened gills

• Labored breathing

• Scratching against objects (flashing)

• Lethargy and loss of appetite

Treatment:

• Manual Removal: For larger parasites like anchor worms, manual removal using sterilized tweezers is effective.

• Medications: Use antiparasitic treatments containing formalin or praziquantel for smaller parasites like epistylis.

• Quarantine: Isolate infected fish to prevent the spread of parasites to healthy individuals.

• Environmental Control: Maintain optimal water quality and reduce stressors to enhance fish immunity.​

Prevention:

• Quarantine new fish before introducing them to the main tank.

• Regularly clean the aquarium and perform water changes.

• Avoid overfeeding and ensure a balanced diet.

16.Popeye Disease (Exophthalmia)

Cause:

Popeye disease is characterized by the swelling and protrusion of one or both eyes in fish. It can result from physical injury, bacterial infections, or poor water conditions.​

Symptoms:

• Swollen, bulging eyes

• Cloudy appearance of the eyes

• Loss of appetite

• Lethargy​

Treatment:

• Water Quality: Improve water conditions by performing regular water changes and ensuring proper filtration.

• Antibiotics: If a bacterial infection is suspected, administer appropriate antibiotics as per veterinary guidance.

• Salt Baths: Using aquarium salt can help reduce swelling and prevent secondary infections.​

Prevention:

• Maintain optimal water parameters.

• Avoid overcrowding and handle fish gently to prevent injuries.

Find out more about Popeye Disease (Exophthalmia).

17. Slime Disease

Cause:

Slime disease is caused by protozoan parasites like Ichthyobodo, Chilodonella, and Trichodina. These parasites irritate the fish's skin, leading to excessive mucus production.​

Symptoms:

• Gray or white mucus coating on the body

• Cloudy eyes

• Scratching against objects

• Clamped fins

• Lethargy​

Treatment:

• Medications: Treat with formalin, copper sulfate, or potassium permanganate as per product instructions.

• Salt Treatment: Gradually increase salinity to 3–5 grams per liter, maintaining for 7–14 days.

• Environmental Control: Enhance water quality and reduce stressors.​

Prevention:

• Regularly clean the aquarium and perform water changes.

• Quarantine new fish before introducing them to the main tank.

18. Swim Bladder Disease

Cause:

Swim bladder disease affects a fish's buoyancy, causing them to float uncontrollably or sink to the bottom. Causes include overfeeding, constipation, infections, or physical deformities.​

Symptoms:

• Difficulty swimming

• Floating at the surface or sinking to the bottom

• Swollen abdomen

• Loss of appetite​

Treatment:

• Fasting: Withhold food for 24–48 hours.

• Dietary Changes: Feed cooked, deshelled peas to alleviate constipation.

• Water Quality: Maintain optimal water conditions.

• Medications: If an infection is suspected, consult a veterinarian for appropriate antibiotics. ​

Prevention:

• Avoid overfeeding and provide a balanced diet.

• Maintain clean water conditions.

• Use sinking pellets to prevent air ingestion during feeding.

Find out more about Swim Bladder Disease

19. Ulcers

Cause:

Ulcers are open sores on a fish's body, often resulting from bacterial infections, injuries, or parasitic infestations.​

Symptoms:

• Red, inflamed sores on the skin

• Loss of scales

• Lethargy

• Loss of appetite​

Treatment:

• Water Quality: Improve water conditions with regular changes.

• Antibiotics: Administer broad-spectrum antibiotics as prescribed by a veterinarian.

• Topical Treatments: Apply antiseptics like povidone-iodine to the affected area.​

Prevention:

• Maintain optimal water parameters.

• Handle fish carefully to prevent injuries.

• Quarantine new fish before introducing them to the main tank.

20.Hemorrhagic Septicemia

​Hemorrhagic Septicemia (HS) is a serious disease affecting both freshwater and marine fish, caused by either bacterial or viral pathogens. In aquaculture, the bacterial form is more common and is primarily caused by Aeromonas hydrophila, Aeromonas salmonicida, and Vibrio anguillarum. The viral form, known as Viral Hemorrhagic Septicemia (VHS), is caused by the VHS virus (VHSV), a novirhabdovirus that affects over 50 species of fish

Cause:

Bacterial HS often arises due to:​

• Poor water quality: High levels of ammonia, nitrites, or low oxygen can stress fish, making them more susceptible.

• Overcrowding: High stocking densities increase stress and facilitate disease transmission.

• Injuries: Physical damage to fish can provide entry points for bacteria.

• Temperature fluctuations: Sudden changes can weaken fish immune systems.​

Viral Hemorrhagic Septicemia (VHS)

VHS is transmitted through:​

• Direct contact: Between infected and healthy fish.

• Contaminated water: The virus can survive in water for at least 14 days.

• Carrier species: Some fish can carry the virus without showing symptoms.

• Equipment and humans: Contaminated gear or hands can spread the virus between tanks or facilities.

Symptoms:

Symptoms can vary but commonly include:​

• External signs:

  • Hemorrhages (bleeding) on the body, fins, and eyes.

  • Bulging eyes (exophthalmia).

  • Swollen abdomen.

  • Darkened coloration.

  • Abnormal swimming behavior, such as spiraling or lethargy.

  • Pale gills indicating anemia.​

• Internal signs:

  • Hemorrhages in internal organs, muscles, and the abdominal cavity.

  • Swollen and discolored kidneys.

  • Accumulation of fluid in the body cavity (ascites).

Treatment:

• Antibiotics: Administered through medicated feed or baths. Common antibiotics include sulfa and furans.

• Supportive care:

  • Improving water quality.

  • Reducing stocking densities.

  • Minimizing stress through proper handling.​

Viral Hemorrhagic Septicemia (VHS)

There is no known cure for VHS. Management focuses on:​

• Preventing spread:

  • Quarantining affected fish.

  • Disinfecting equipment and facilities.

  • Restricting movement of fish and eggs from affected areas.

• Biosecurity measures:

  • Using pathogen-free water sources.

  • Implementing strict hygiene protocols for staff and equipment.

Prevention:

• Maintaining optimal water quality: Regular monitoring and adjustments to parameters like pH, ammonia, nitrite, and oxygen levels.

• Proper stocking densities: Avoiding overcrowding to reduce stress and disease transmission.

• Quarantine protocols: Isolating new or sick fish to prevent introducing pathogens.

• Regular health monitoring: Observing fish for early signs of disease.

• Vaccination: Where available, vaccines can provide protection against specific bacterial pathogens.

21.Velvet disease

Cause:

Velvet disease often arises due to:​

• Introducing infected fish without proper quarantine.

• Stressful conditions, such as poor water quality or sudden temperature changes.

• Overcrowding and inadequate tank maintenance.

• Use of contaminated equipment or live foods.

The parasites thrive in environments where fish are stressed or immunocompromised.

Symptoms:

• Visual inspection for characteristic gold dusting.

• Microscopic examination of skin or gill scrapings to identify parasites.

• Observation of behavior and respiratory distress.​

Consulting an aquatic veterinarian is recommended for accurate diagnosis and treatment planning.

Treatment:

Effective treatment targets the free-swimming stage of the parasite:​

• Copper-Based Medications: Use chelated copper treatments like Mardel Coppersafemfor 10 days. Monitor copper levels to avoid toxicity.​

• Aquarium Salt: Adding non-iodized salt can help reduce stress and support gill function.​

• Temperature Adjustment: Gradually raise water temperature to 82°F (28°C) to speed up the parasite's life cycle, making it more susceptible to treatment.​

• Light Reduction: Dim or turn off aquarium lights for several days, as the parasites rely on light for photosynthesis.​

• Alternative Treatments: In freshwater systems, herbal remedies like Microbe-Lift Herbtana may be effective.​

Important: Remove activated carbon from filters during treatment, as it can absorb medications.

Prevention:

• Quarantine New Additions: Isolate new fish for 2–4 weeks before introducing them to the main tank.​

• Maintain Water Quality: Regularly test and adjust water parameters to optimal levels.​

• Avoid Overcrowding: Provide adequate space to reduce stress and disease transmission.​

• Sanitize Equipment: Clean nets, siphons, and other tools between uses.​

• Feed Nutritious Diets: Provide a balanced diet to strengthen fish immune systems.

Find out more about velvet in fish