15 Molly Fish Diseases & Their Treatments: A Complete Guide

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I love growing molly fish in my tank. Let’s face it, these fish are extremely beautiful. However, when they seem sick or feel unwell, I know that it’s my responsibility to take care of them.

Unfortunately, diagnosing diseases in mollies can be pretty challenging, especially because many of them come with similar symptoms.

That’s why I have compiled this guide, which covers 15 diseases that can affect molly fish. 

For each one, you will learn how to identify the condition, how to treat it, and how to prevent it from recurring in the future.

Let’s get started.

Also Read: Molly Fish Care Guide

White Spot Disease/IchWhite spots on body, Clamped fins, Scratching against surfaces, Lethargy and loss of appetiteIncrease water temperature, Salt treatment, Medicated baths, Anti-Ich medication, Quarantine infected fish
Fin & Tail RotRagged or punctured fins, Change in color, Incremental degenerationAnti-bacterial or anti-fungal treatments, Improve water condition, Vitamin supplements, Quarantine infected fish
Swim Bladder DisorderDifficulty swimming, Abnormal behavior, Less activeChange diet, Lower water depth, Medication, Fasting
Velvet (Oodinium)Velvety coat, Scratching behavior, Clamped fins and lethargyAnti-parasitic medication, Increased temperature, Darken the environment, Quarantine infected fish
ColumnarisCotton-like patches, Altered breathing and diminished activity, Reduced food intakeUse of antibacterial medication, Improving water quality, Raising the temperature, Salt treatment
Gill FlukesRapid gill movement, Scratching behavior, Red or swollen gillsAnti-parasitic medication, Salt bath, Increase water temperature, Quarantine infected fish
DropsyBloating or swelling, Pinecone-like scales, Lethargy and loss of appetiteAntibacterial medication, Epsom salt baths, Improve water quality, Isolate infected fish
Black Spot DiseaseAppearance of black spots, Scratching behavior, Reduced appetite and lethargyAnti-parasitic medication, Quarantine infected fish, Tank cleaning and water change
ConstipationBloating, Less or no fecal matter, Swimming difficultyDiet change, Fasting, Epsom salt bath
Scoliosis (Bent Spine)Bent or curved spine, Swimming difficulty, General weaknessImproved diet, Antibiotic treatment, There’s no guaranteed cure
Fish TuberculosisWeight loss and lethargy, Loss of scales and skin color, Lesions and ulcersAntibiotics, Isolate infected fish, Euthanasia in severe cases
Popped EyesProtruding eyes, Cloudy eyes, Swimming difficulty or lethargyWater change, Antibiotic treatment, Isolate infected fish
Camallanus Internal WormVisible worms, Weight loss and decreased appetite, LethargyAnti-parasitic medication, Water changes and tank cleaning, Quarantine infected fish
Viral Hemorrhagic Septicemia (VHS)Hemorrhaging, Decreased activity and appetite, Protruding eyes or bloated bellyNo established cure, Isolate infected mollies, Supportive measures
Swollen GillsEnlargement or redness of gills, Accelerated gill movement, Diminished appetite or lack of energyChange of water and evaluation of quality, Application of antibiotics or anti-parasite treatment, Segregation of infected fish

1. White Spot Disease/Ich

White Spot Disease, also known as Ich, is a common parasitic infection seen in molly fish and other tropical freshwater species.

Ich is caused by the protozoan parasite Ichthyophthirius multifiliis, which can proliferate quickly in aquarium conditions, causing various health issues for mollies and other fish.


  • White spots on body: The presence of tiny, white, salt-like spots on the molly fish’s skin, fins, and gills are a hallmark of Ich.
  • Clamped fins: The molly fish may adopt a defensive stance, keeping its fins close to its body, indicating discomfort or stress.
  • Scratching against surfaces: The molly fish might be seen constantly rubbing against aquarium decorations, an attempt to relieve the irritation caused by Ich.
  • Lethargy and loss of appetite: A noticeable decrease in activity and refusal to eat are common indications of declining health in an Ich-infected molly fish.


  • Increase water temperature: Gradually increase the aquarium’s temperature to around 86°F (30°C), which speeds up the life cycle of the Ich parasite and aids in its elimination.
  • Salt treatment: Adding 1-2 teaspoons of aquarium salt per gallon of water can help mollies fight off the Ich parasite by enhancing their natural protective slime coating.
  • Medicated baths: Bathing the molly fish in a separate container with anti-parasitic medications, such as malachite green or methylene blue, can directly target the Ich parasites.
  • Anti-Ich medication: Treat the entire tank with a specifically formulated anti-Ich medication like copper sulfate, following the dosage instructions provided by the manufacturer. My recommendation: Fritz Mardel (link to Amazon).
  • Quarantine infected fish: It’s crucial to immediately separate infected mollies and place them in a quarantine tank to stop the disease from spreading to other fish.


  • Regular water changes: Carry out water changes of 10-20% weekly to maintain cleanliness and minimize the likelihood of Ich proliferation.
  • Quarantine new fish: Before introducing new fish to the tank, keep them in a separate quarantine tank for about 2-4 weeks to observe for any signs of Ich or other diseases.
  • Avoid overcrowding: A tank with ample space helps reduce stress among molly fish, making them less vulnerable to diseases like Ich. Stick to the guideline of one gallon of water per inch of fish to prevent overcrowding.

Also Read: Molly Fish Ich

2. Fin & Tail Rot

Molly fish are prone to a condition known as fin and tail rot, typically caused by bacterial or fungal infections.

These infections usually arise due to suboptimal water conditions or physical injuries.

Early detection is crucial, as, if ignored, the disease can progressively decay the fins and tail, and ultimately spread to the fish’s body.


  • Ragged or punctured fins: The fins and tail of the molly fish might look ragged or broken, deviating from their usual well-preserved state.
  • Change in color: Signs of infection might include a white or red hue along the borders of the fins and tail.
  • Incremental degeneration: Without intervention, the condition of the fins and tail may progressively deteriorate, potentially causing a systemic infection in the fish.


  • Employ suitable anti-bacterial or anti-fungal treatments: Use aquarium-safe drugs like Melafix or Pimafix, ensuring to follow the guidelines provided by the manufacturer. My recommendation: API MELAFIX (link to Amazon).
  • Improve water condition: Periodically replace the water (around 10-20% weekly) and keep proper filtration to sustain a sanitary habitat conducive to recovery.
  • Offer vitamin supplements: Use vitamin supplements to strengthen the immune response of the molly fish, helping in convalescence and fin regeneration.
  • Quarantine infected fish: Relocate the sick fish to a different tank to avoid the spread of the disease among other aquarium inhabitants.


  • Periodic water replacements: Keep the water pristine by changing it frequently, ensuring to meet the appropriate water parameters for mollies.
  • Evade physical injuries: Avoid installing sharp or rough aquarium decor that might injure the fish, thus increasing their susceptibility to infections.
  • Regulate aggression: Watch out for aggressive fish that may harm others by biting or nipping, leading to injuries that can easily develop into fin rot.

Also Read: Molly Fish Fin Rot

3. Swim Bladder Disorder

Swim Bladder Disorder is a condition that affects the molly fish’s swim bladder, an internal organ responsible for maintaining buoyancy.

It often results from dietary problems or water conditions, causing the fish to lose control of its buoyancy, leading to difficulties in swimming.


  • Difficulty swimming: The molly fish may struggle to maintain a normal position in the water, possibly swimming nose-up, nose-down, or even on its side.
  • Abnormal behavior: An affected molly fish may float at the surface of the water or sink to the bottom, indicative of problems with maintaining its buoyancy.
  • Less active: Lethargy and decreased levels of activity are commonly exhibited by molly fish affected by Swim Bladder Disorder.


  • Change diet: Switch to a diet of peas, which work as a gentle laxative, to relieve any constipation that might be putting pressure on the swim bladder.
  • Lower water depth: Reducing the depth of the water can help alleviate pressure on the swim bladder, making it easier for the fish to swim.
  • Medication: In cases of bacterial infection causing the disorder, use suitable antibacterial medicines, always following manufacturer guidelines.
  • Fasting: A brief period of fasting (24-48 hours) can help alleviate potential constipation, often a cause of Swim Bladder Disorder.


  • Proper diet: Feed mollies a balanced and varied diet to avoid constipation, and consider soaking dry foods before feeding to prevent them from expanding in the fish’s stomach.
  • Good water quality: Regular water changes and maintenance of suitable water conditions can prevent infections that might lead to Swim Bladder Disorder.
  • Avoid overfeeding: Overfeeding can lead to constipation, which can negatively affect the swim bladder. Aim for feeding small amounts 2-3 times a day.

Also Read: Molly Fish Swim Bladder Disease

4. Velvet (Oodinium)

Velvet, also known as Oodinium, is a parasitic skin disease in molly fish caused by dinoflagellates.

The parasites multiply on the fish’s skin, causing a velvety, dusty appearance that can lead to severe stress and damage if left untreated.


  • Velvety coat: Molly fish may display a velvety, gold, or rust-colored dust on their skin, hence the name ‘velvet’.
  • Scratching behavior: Affected mollies may scratch against objects in the tank due to skin irritation.
  • Clamped fins and lethargy: Mollies may show signs of discomfort like clamping their fins close to their bodies and reduced activity levels.


  • Anti-parasitic medication: Administer an anti-parasitic medication, such as copper sulfate, to the tank according to the guidelines provided by the manufacturer. My recommendation: Fritz Mardel (link to Amazon).
  • Increased temperature: Increase the water temperature to approximately 80-82°F (26-28°C) in order to accelerate the parasite’s life cycle, thereby enhancing the effectiveness of the treatment.
  • Darken the environment: Parasites causing Velvet are photosynthetic. Keeping the tank in a dark environment for several days can aid in treatment.
  • Quarantine infected fish: If possible, isolate affected fish in a separate tank to prevent further spread of the disease.


  • Regular water changes: Maintain good water quality with frequent water changes to reduce the risk of a Velvet outbreak.
  • Quarantine new arrivals: Isolate new fish for at least 2 weeks before introducing them to the main tank to avoid introducing Velvet parasites.
  • Avoid overcrowding: Overcrowded conditions can stress fish and make them more susceptible to infections like Velvet.

Also Read: Stress In Molly Fish

5. Columnaris

Columnaris, also known as Flavobacterium columnare, is a bacterial disease that primarily affects molly fish, inflicting damage to their skin, gills, and mouth, and causing a cotton-like appearance.

The condition can be lethal if not addressed promptly.


  • Cotton-like patches: Mollies affected by Columnaris may have white, fluffy spots on their skin, gills, or mouth.
  • Altered breathing and diminished activity: Mollies suffering from this condition may display fast-paced breathing and show a reduction in their usual activity.
  • Reduced food intake: The fish might also show a decreased interest in feeding, indicating an overall health deterioration.


  • Use of antibacterial medication: Broad-spectrum antibiotics such as Kanamycin or Tetracycline can be used as per the manufacturer’s guidelines. My recommendation: API MELAFIX (link to Amazon).
  • Improving water quality: Regular water replacements and adequate filtration can assist in the recovery process by removing harmful bacteria.
  • Raising the temperature: A slight increase in water temperature can stimulate the molly fish’s immune system.
  • Salt treatment: Implementing a gentle salt treatment with aquarium salt can reduce stress and boost the fish’s overall immunity.


  • Regular water replacements: Keeping the water clean through routine changes can lessen bacterial presence.
  • Avoid overpopulation: An overcrowded tank can stress mollies, making them more prone to bacterial diseases.
  • Balanced diet and suitable habitat: Offering a nutritious diet and establishing an environment tailored to the mollies’ natural needs are essential.

6. Gill Flukes

Gill Flukes is a parasitic disease in molly fish, caused by Dactylogyrus (gill flukes) and Gyrodactylus (skin flukes).

These microscopic worms attach to the gills, skin, or fins of the fish, causing irritation and potentially severe damage.


  • Rapid gill movement: Mollies infected with gill flukes may display rapid or labored breathing due to gill irritation.
  • Scratching behavior: Fish may scratch against objects in the tank due to discomfort caused by the parasites.
  • Red or swollen gills: Affected mollies might exhibit visibly reddened or swollen gills.


  • Anti-parasitic medication: Treat the aquarium with a fluke-specific medication like Praziquantel, following the instructions provided by the manufacturer. My recommendation: Hikari Prazipro (link to Amazon).
  • Salt bath: A brief salt bath using 2-3 teaspoons of aquarium salt per gallon of water can help dislodge the flukes.
  • Increase water temperature: Slightly raising the aquarium’s temperature can speed up the lifecycle of the parasites, making treatment more effective.
  • Quarantine infected fish: Isolating affected mollies can prevent the spread of the disease to other fish in the tank.


  • Regular water changes: Keep the aquarium clean with regular water changes to reduce the likelihood of parasitic outbreaks.
  • Quarantine new arrivals: Isolate new fish in a separate tank for at least 2 weeks before introducing them to the main tank to avoid introducing new parasites.
  • Regular health checks: Regularly monitor mollies for signs of infection or changes in behavior to catch and treat any disease early.

7. Dropsy

Dropsy is a condition in molly fish characterized by a swollen, pinecone-like appearance due to fluid accumulation.

It’s often a sign of kidney failure or severe bacterial infection and can be difficult to treat if not caught early.


  • Bloating or swelling: Mollies with dropsy often exhibit noticeable swelling or bloating in their bodies.
  • Pinecone-like scales: The scales of the fish may protrude outward, giving the fish a pinecone-like appearance.
  • Lethargy and loss of appetite: Affected mollies might display reduced activity and decreased interest in food.


  • Antibacterial medication: Treat the tank with a broad-spectrum antibiotic such as Kanamycin or Maracyn-Two to combat potential bacterial infections.
  • Epsom salt baths: Epsom salt baths can help reduce swelling by drawing out excess fluids. Use about 1 tablespoon of Epsom salt per 5 gallons of water.
  • Improve water quality: Regular water changes can help create a cleaner environment that aids in recovery.
  • Isolate infected fish: To prevent potential spread of any underlying bacterial infections, isolate affected fish in a separate quarantine tank.


  • Good water quality: Maintaining clean water with regular changes can prevent the buildup of harmful bacteria.
  • Healthy diet: Provide a balanced diet to boost the immune system of mollies, helping to ward off diseases like dropsy.
  • Regular health checks: Regularly monitor the mollies for signs of illness to catch any disease in its early stages.

Also Read: Pregnant Molly Fish

8. Black Spot Disease

Diplopstomiasis, commonly referred to as Black Spot Disease, is a parasitic infection that affects molly fish.

This condition is characterized by the presence of small black spots on the fish’s skin and is usually caused by flatworm larvae.

It is more prevalent in mollies that reside in outdoor ponds.


  • Appearance of black spots: The molly fish’s skin, fins, and occasionally the gills develop tiny black spots.
  • Scratching behavior: Infected mollies may exhibit rubbing or scratching against the aquarium decorations or substrate due to skin irritation.
  • Reduced appetite and lethargy: Affected mollies may show a decrease in appetite and reduced activity levels.


  • Anti-parasitic medication: Administer suitable anti-parasitic medication as directed by the manufacturer’s instructions. My recommendation: Hikari Prazipro (link to Amazon).
  • Quarantine infected fish: Isolate the infected fish in a separate tank to prevent the spread of parasites.
  • Tank cleaning and water change: Thoroughly clean the tank and perform a significant water change to eliminate larvae and reduce parasite populations.


  • Regular water changes: Maintain a clean environment by regularly changing the water, which helps limit parasitic outbreaks.
  • Control snail populations: Snails can serve as intermediate hosts for the parasite. Managing snail populations can aid in disease prevention.
  • Quarantine new fish: Isolate newly acquired fish for a minimum of two weeks in a separate tank before introducing them to the main tank to prevent the introduction of parasites.

Also Read: Black Spots On Molly Fish

9. Constipation

Constipation in molly fish often results from a diet too rich in protein and lacking in fiber. It can lead to difficulty in swimming and overall discomfort.


  • Bloating: Mollies suffering from constipation may show signs of abdominal swelling or bloating.
  • Less or no fecal matter: Infected fish may produce less waste than usual, or none at all.
  • Swimming difficulty: Affected fish might have trouble swimming normally due to discomfort and bloating.


  • Diet change: Switching to a diet high in fiber can help relieve constipation. Peas are an excellent choice as they act as a natural laxative.
  • Fasting: A short period of fasting (24-48 hours) can help clear the digestive system.
  • Epsom salt bath: An Epsom salt bath can help draw out excess fluids and relieve swelling, using about 1 tablespoon per 5 gallons of water.


  • Balanced diet: Feed a balanced diet with adequate fiber content to prevent constipation.
  • Avoid overfeeding: Overfeeding can lead to digestive issues. Feed small amounts several times a day instead of a large quantity at once.
  • Provide variety in diet: Providing a variety of food can help ensure a balanced intake of nutrients and fiber.

Also Read: Molly Fish Constipation

10. Scoliosis (Bent Spine)

Scoliosis, or bent spine, in molly fish can be caused by genetic factors, poor nutrition, or bacterial infections.

It manifests as a noticeable curve in the fish’s spine, which can affect swimming and overall health.


  • Bent or curved spine: The most noticeable symptom is a visible bend or curve in the molly fish’s spine.
  • Swimming difficulty: Mollies with scoliosis may swim in a peculiar, often erratic manner due to the spinal deformation.
  • General weakness: Affected fish may show signs of overall weakness and decreased activity levels.


  • Improved diet: If scoliosis is caused by nutritional deficiencies, improving the diet can sometimes help. Ensure a balanced diet with high-quality, varied foods.
  • Antibiotic treatment: If a bacterial infection is suspected, consult a vet for appropriate antibiotic treatment. My recommendation: API MELAFIX (link to Amazon).
  • There’s no guaranteed cure: Unfortunately, if scoliosis is genetic or has advanced significantly, there might not be an effective treatment.


  • Healthy diet: Feeding a nutritionally balanced, varied diet can help prevent deficiency-related scoliosis.
  • Careful breeding: Avoid breeding mollies with known spinal issues to prevent passing on genetic conditions.
  • Regular health checks: Regularly inspect mollies for signs of abnormal curvature to catch potential issues early.

11. Fish Tuberculosis

Fish Tuberculosis (Mycobacteriosis) is a serious bacterial infection in molly fish caused by Mycobacterium marinum.

It’s a slow-progressing disease and can cause a range of symptoms from weight loss to lesions.


  • Weight loss and lethargy: Infected mollies might lose weight noticeably and become less active.
  • Loss of scales and skin color: Affected fish may lose scales or their skin color may fade.
  • Lesions and ulcers: In advanced cases, the fish might develop visible lesions or ulcers on their bodies.


  • Antibiotics: Consult with a vet for a suitable antibiotic regimen. Treatment often involves a long course of specific antibiotics.
  • Isolate infected fish: Infected fish should be isolated to prevent the spread of the bacteria to other tank mates.
  • Euthanasia in severe cases: In severe or advanced cases, euthanasia might be the most humane option to prevent suffering.


  • Maintain good water quality: Regular water changes and good filtration can help limit the spread of the bacteria.
  • Avoid overcrowding: Stress from overcrowding can weaken mollies’ immune systems, making them more susceptible to infection.
  • Regular health checks: Regularly check your fish for signs of disease, and quarantine any new arrivals before adding them to your tank.

12. Popped Eyes

Popped Eyes, also known as Exophthalmia, is a condition where one or both eyes of the molly fish protrude from the sockets.

It’s usually a symptom of underlying issues, like bacterial infections, parasitic infections, or poor water quality.


  • Protruding eyes: The most noticeable symptom is one or both eyes of the molly fish sticking out more than normal.
  • Cloudy eyes: In some cases, the protruding eyes might also become cloudy or discolored.
  • Swimming difficulty or lethargy: Affected mollies might swim abnormally or become less active.


  • Water change: Perform a 50% water change to ensure the fish is in a clean environment while you diagnose the issue.
  • Antibiotic treatment: If you think your fish has a bacterial infection, you can use a broad-spectrum antibiotic like Erythromycin. My recommendation: API MELAFIX (link to Amazon).
  • Isolate infected fish: To prevent potential spread of any underlying bacterial infections, isolate affected fish in a separate quarantine tank.


  • Maintain water quality: Regular water changes and good filtration can prevent infections that lead to popped eyes.
  • Nutritious diet: A balanced diet can help boost the mollies’ immune system, making them less susceptible to infections.
  • Regular health checks: Monitor your mollies regularly for signs of illness and take action quickly if popped eyes or other symptoms occur.

Also Read: Molly Fish With Bulging Eyes

13. Camallanus Internal Worm

Camallanus is a genus of parasitic nematodes that can infect molly fish, causing significant internal damage.

They are particularly dangerous as symptoms often only show when the infestation is quite advanced.


  • Visible worms: Reddish-brown worms might be visible protruding from the molly fish’s anus.
  • Weight loss and decreased appetite: Infected mollies may lose weight noticeably and show less interest in food.
  • Lethargy: Affected fish might become less active and show signs of overall weakness.


  • Anti-parasitic medication: Treat with an anti-parasitic medication such as Levamisole or Fenbendazole, following the instructions provided by the manufacturer. My recommendation: Hikari Prazipro (link to Amazon).
  • Water changes and tank cleaning: Regular water changes and thorough cleaning of the tank can help remove any remaining worms and eggs.
  • Quarantine infected fish: Separate infected fish to prevent the spread of the parasites.


  • Quarantine new arrivals: Isolate new fish in a separate tank for at least 2 weeks before introducing them to the main tank to avoid introducing parasites.
  • Good water quality: Regular water changes and good filtration can help limit the chances of parasitic outbreaks.
  • Regular health checks: Regularly inspect your mollies for signs of parasites to catch potential infestations early.

14. Viral Hemorrhagic Septicemia (VHS)

Viral Hemorrhagic Septicemia (VHS) is an intense viral ailment that can impact molly fish.

It is marked by internal hemorrhaging, which can result in anemia and, in extreme instances, fatality. This highly infectious disease has the potential to quickly proliferate in an aquarium.


  • Hemorrhaging: Mollies infected with VHS might display evidence of internal bleeding, such as bloody lines on their bodies or underneath their skin.
  • Decreased activity and appetite: Fish afflicted with the disease may demonstrate lethargy and diminished enthusiasm for food.
  • Protruding eyes or bloated belly: When the disease progresses, symptoms may include eyes that bulge or an abdomen that enlarges due to internal harm and fluid build-up.


  • No established cure: Regrettably, there is no recognized treatment for VHS as of now.
  • Isolate infected mollies: Immediately sequester fish with the infection to inhibit the virus from contaminating other aquarium residents.
  • Supportive measures: Supply supportive treatment, like ensuring water cleanliness and alleviating stress, to bolster the fish’s innate immune defenses.


  • Quarantine newly acquired fish: Make sure to isolate new fish for at least two weeks prior to introducing them to the primary tank to ward off viral transmission.
  • Routine wellness evaluations: Regularly inspect your mollies for disease symptoms to catch potential problems early.
  • Stress reduction: Uphold a consistent, stress-free habitat for your mollies, as stress degrades their immune systems, increasing their susceptibility to viral diseases.

15. Swollen Gills

Molly fish exhibiting swollen gills might be suffering from various potential issues such as bacterial or parasitic afflictions, or they might have been subjected to suboptimal water environments, like excessive ammonia concentrations.


  • Enlargement or redness of gills: The gills of the affected molly fish might show noticeable swelling or a reddened hue.
  • Accelerated gill movement: Fish that are infected might show signs of quick or strained respiration due to gill discomfort.
  • Diminished appetite or lack of energy: Fish who have swollen gills might consume less food and demonstrate a drop in their activity levels.


  • Change of water and evaluation of quality: If poor water quality is suspected, perform an immediate water change and conduct tests to identify any potential imbalances, such as elevated ammonia levels.
  • Application of antibiotics or anti-parasite treatment: Depending on the suspected root cause, administer the relevant medication to tackle the infection or parasitic issue.
  • Segregation of infected fish: To prevent potential transmission of pathogens, isolate the infected fish from the rest of the tank occupants.


  • Preservation of water quality: To prevent conditions that could cause gill swelling, regularly execute water changes and keep an eye on water parameters.
  • Routine health inspections: Regularly keep tabs on your molly fish for any alterations in their behavior or physical condition.
  • Isolation of new additions: Before introducing new fish to your main tank, quarantine them in a separate tank for a minimum of 2 weeks to prevent pathogen introduction.


If you think that your molly fish is sick, the first step would be to quarantine it in a dedicated tank to prevent the disease from spreading.

Next, consult an aquatic veterinarian. An expert will diagnose and treat your fish’s condition in the most accurate and professional way.

In the meantime, you can try applying a treatment yourself using this guide and some commercial products available online.