Molly Fish Diseases, Parasites & Remedies

It can be deeply disheartening to see a beloved pet fall ill, particularly when you’re uncertain about the cause of the illness and the appropriate steps to take for treatment. Like all animals, molly fish are susceptible to various diseases.


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Understanding the diseases that commonly affect mollies, and knowing how to prevent and treat these conditions, can empower you to manage health issues effectively when they arise.

Many of the most frequent diseases in molly fish, such as ich, fin rot, flukes, or velvet, can often be treated with medications like Seachem ParaGuard. This medication has proven to be very effective.

In the following sections, you’ll find detailed information about the most common diseases, parasites, and remedies for molly fish. Additionally, I’ll provide practical tips on disease prevention in mollies.

My goal is to offer a comprehensive overview of these diseases, including their symptoms and potential treatments, so you’ll be better prepared to handle any health issues your molly fish might encounter.

Symptoms and Treatments for Molly Fish Diseases and Parasites

These are the diseases and illnesses you’re most likely to encounter in a molly fish tank, so take notice of their symptoms and treatment.

1. White Spots (Ich or Ick)


Molly Fish Infested with Ich (White Spots)

Ich or Ick, easily recognizable by the white spots on the skin and fins of molly fish, is a disease caused by an ectoparasite. It is treatable if addressed promptly.

In addition to the characteristic spots, infected fish often exhibit behavior like rubbing themselves against objects in the tank, an attempt to relieve the irritation caused by the spots. The disease may also lead to a loss of appetite.

Treatment of White Spots:

  • Temperature Adjustment: Gradually increase the tank water temperature to 80°F (about 27°C) to accelerate the parasite’s life cycle and make it more susceptible to treatment.
  • Medication: Add Seachem ParaGuard to the tank following the dosage instructions on the label.
  • Aquarium Salt: Alternatively, you can add aquarium salt at a rate of 1 teaspoon per gallon of water.
  • Duration: Maintain the increased temperature and treatment for 4-7 days.
  • Water Change and Cleaning: After the treatment period, perform a major water change, at least 70%, and thoroughly clean the substrate to remove any remaining parasites or eggs.
  • Whole Tank Treatment: It’s important to treat the entire tank, not just a separate hospital tank, to ensure all traces of the parasite are eradicated.

2. Velvet


Molly Fish with Velvet Disease

Velvet disease, caused by the Oodinium parasite, leads to the formation of small, gold-colored cysts on the skin of fish. This disease can spread rapidly and have severe consequences.

Early detection is key for effective treatment, but this can be challenging as the cysts are very small initially. As the disease progresses, more noticeable lesions may appear on the skin.

Treatment of Velvet Disease:

  • Copper-Based Medication: In the early stages, copper-based medications, such as Seachem Cupramin, have been shown to be effective. Follow the dosage instructions carefully.
  • Reducing Light: Turning off aquarium lights during treatment can be beneficial, as the Oodinium parasite is photosynthetic. Keep the lights off until the disease has been fully eradicated.
  • Water Change and Cleaning: After the symptoms have subsided, perform a major water change, between 70% to 90%. This helps to remove any remaining parasites and improves overall water quality.
  • Tank Environment: Ensure the water quality is optimal and reduce stressors for the fish during and after treatment.

3. Fin and Tail Rot

Fin and tail rot in molly fish can be caused by either fungal or bacterial infections. These infections often thrive in tanks with poor conditions, such as spikes in ammonia levels.

Injuries to fish in suboptimal tank conditions can become gateways for opportunistic bacteria and fungi.

Signs of fin and tail rot include fins that appear chewed, shredded, or clumped together. In advanced stages, white, milky patches may appear on other parts of the body.

Treatment for Fin and Tail Rot:

  • Quarantine Affected Fish: Isolate infected fish to prevent the spread of the disease.
  • Antibiotic Treatment: For bacterial infections, start a course of antibiotics targeting gram-negative bacteria, such as Maracyn, Maracyn 2, or Tetracycline. Seachem ParaGuard can also be an effective alternative.
  • Fungal Treatment: Use appropriate medication for fungal infections.
  • Water Change: Perform a 20-50% water change in the original tank to improve water quality and reduce the presence of pathogens.

Note on Recovery:

  • Be aware that while fish can recover from fin and tail rot, severely damaged fins or tails may not fully regrow.

4. Protozoan Infection

Protozoan parasites can flourish in poor water conditions, posing a significant risk to fish. These parasites can enter a fish’s system through cuts or scrapes, or by attaching themselves to the fish’s skin. Over time, they can penetrate deeper into the muscles and eventually enter the bloodstream.

Common signs of a protozoan infection include small white specks on the skin, excessive slime production, loss of appetite, and lethargy.

Treatment of Protozoan Disease:

  • Stable Water Temperature: Maintaining a stable water temperature is crucial for preventing and managing protozoan infections. Using an aquarium heater with a thermostat can help achieve this.
  • Medication: In the early stages of infection, treatments with Malachite Green or Formalin are effective. For more advanced stages, copper-based medications may be necessary.
  • Water Change: After completing the treatment, perform a water change of 50-70% to improve water quality and remove any residual medication and parasites.

5. Mouth Fungus & Columnaris


Molly Fish with Columnaris

Mouth fungus, often mistaken for a fungal infection due to its appearance, is actually caused by bacteria. It manifests as white, thread-like growths or patches on the mouth or body of the fish. This condition may also present with ragged fins, rapid breathing, excessive mucus production, and, in advanced stages, sores and lesions.

Prompt treatment is essential for the best outcome.

Treatment of Mouth Fungus Disease:

  • Antibiotic Treatment: Use antibiotics like Maracyn or treatments containing Formalin. Follow the dosage instructions carefully.
  • Aquarium Salt: Add aquarium salt to the tank at a concentration of one teaspoon per gallon, once daily for three days. This can aid in the healing process.
  • Water Change: Once the symptoms have subsided, conduct a 50-70% water change to remove lingering bacteria and improve water quality.
  • Potassium Permanganate Dip: As an alternative treatment, you can prepare a dip solution with 10 mg/l of potassium permanganate. Submerge the fish in this solution for no more than 30 minutes. Caution is advised, as incorrect dosage or prolonged exposure can harm the fish.

6. Dropsy

Dropsy is a severe condition in fish that, unfortunately, is often untreatable. It is not a disease in itself but a symptom of underlying issues, typically involving kidney or liver dysfunction, leading to fluid retention.

This fluid retention causes a bloated appearance, with scales that stand out from the body, resembling a pinecone. Affected fish may also exhibit swimming difficulties. Poor water quality and stress are common contributors to the development of dropsy.

Treatment of Dropsy:

  • Limited Treatment Options: Once symptoms of dropsy appear, it’s often too late for effective intervention, and the condition is usually irreversible.
  • Epsom Salt Baths: While not a cure, Epsom salt baths can help alleviate some symptoms and provide comfort to the fish. These baths can help reduce fluid retention, but they do not address the underlying cause of dropsy.
  • Preventative Measures: Focus on maintaining excellent water quality and minimizing stress in the aquarium to help prevent dropsy. Regular water testing and proper tank management are essential.

7. Swollen Gills

If you notice swollen gills on your molly fish or if your mollies coming to the surface and gasping for air, it’s a sign of bad quality water in the aquarium.

Ammonia or carbonate poisoning is known to cause swollen gills and cause fish to gasp for air.

Irregular water changes, food left to decay in the aquarium can all cause ammonia levels to spike.

Treatment for swollen gills:

  • Do a 50% water change and monitor toxin levels over the next few days;
  • Don’t feed your mollies for a couple of days until swelling subsides and to prevent further fouling of water;
  • Add some nitrifying bacteria to the aquarium (you can use API Quick Start).

8. Red Blood Spot


Molly Fish – Red Blood Spots

Red Blood Spots typically appear in newly cycled tanks where water chemistry is imbalanced. These spots, manifesting on the stomach or body of your molly fish, can indicate ammonia or nitrite poisoning.

In aquariums that are not properly cycled, high levels of ammonia and nitrites are common. If you are a beginner, it’s crucial to familiarize yourself with the nitrogen cycle process. Let your aquarium stabilize for at least 2 weeks, though 6 weeks is preferable, before introducing any mollies.

Treatment of Red Blood Spot:

Regrettably, this condition is often untreatable and can lead to the eventual demise of your fish. Prevention, through proper execution of the nitrogen cycle, is key.

In the disease’s very early stages, there might be a chance to save your fish, or at least some of them, by immediately taking measures to reduce aquarium toxins. This can include skipping feeding for a day and performing a water change.

Continue to monitor ammonia and nitrite levels regularly. Performing consistent water changes helps dilute toxins that accumulate from fish waste.

9. Viral Haemorrhagic Septicaemia

Viral Hemorrhagic Septicemia is a severe viral infection in fish, characterized by symptoms such as lesions, ulcers, pale gills, bulging eyes, and, as the disease progresses, fin rot. Infected fish often exhibit a loss of appetite, a darker coloration, and unfortunately, the condition often leads to death.

Treatment of VHS:

  • Antibiotic Treatment: At the first signs of VHS, antibiotics like Maracyn 2 or API Furan 2 can be used. It’s important to note that while these treatments may help manage symptoms, they do not directly treat the virus, as antibiotics are ineffective against viral infections.
  • Water Change: Perform a significant water change after the treatment course to help improve the overall water quality and reduce stress on the fish.

10. Swim Bladder Disorder & Inflammation

The swim bladder is a gas-filled internal organ that plays a crucial role in maintaining a fish’s buoyancy, balance, and stability at various water depths.

Swim Bladder Disorder:

  • While not a frequent issue in mollies, swim bladder disorder can occur due to high stress or poor water quality. Symptoms include difficulty maintaining buoyancy, swimming at unusual angles, or floating/sinking abnormally.

Swim Bladder Inflammation:

  • Contrary to swim bladder disorder, swim bladder inflammation is caused by a viral infection and is typically incurable. Affected fish often exhibit a distended belly and may swim with their head pointing downward.
  • It is advisable to isolate fish showing symptoms of swim bladder inflammation to prevent the potential spread of the virus.
  • Humane euthanasia may be considered for fish with severe symptoms, as the condition is incurable and can lead to suffering.

11. Pop-Eye Disease

Pop-eye in fish, characterized by the protrusion of the eyes, is not a disease in itself but a symptom that can arise from various underlying conditions, such as fungal or bacterial infections, dropsy, tuberculosis in fish, or internal parasites.

Identifying the root cause can be challenging, making treatment difficult. Pop-eye is not typically fatal, but it can be distressing for the fish, as it causes their eyes to bulge, and in severe cases, the eyes may even become dislodged.

Treatment of Pop-Eye:

  • Identify the Cause: If you can pinpoint a specific underlying condition (such as a bacterial or fungal infection), address it with the appropriate treatment.
  • General Care: Improve general care and water quality, as poor conditions can exacerbate or contribute to the development of Pop-eye.
  • Isolation: Consider isolating the affected fish to prevent potential stress from other fish and to monitor its condition closely.

Molly Fish with Pop-Eye


Pop Eye Disease – Sailfin Molly Fish

12. Gill Flukes

Gill flukes is a parasitic infection caused by a tiny white worm that burrows itself into the gills of your fish causing breathing difficulty and bleeding.

Molly fish with gill flukes can be seen gasping for air at the surface of the tank or in the bottom of the tank.

Gill worms are usually introduced into the aquarium via new plants or new fish, so make sure you always quarantine new fish.

Treatment of gill flukes:

  • In its early stages, special medication to treat gill flukes is available and you should do a tank level treatment. More advanced cases, i.e., when bleeding already occurs cannot be treated.

13. Camallanus Parasite

The Camallanus parasite, typically brown or orange in color, is often visible protruding from the anus of infected fish. Introducing new fish to an aquarium, particularly those previously housed in ponds, can be a common way for this parasite to spread.

Treatment of the Camallanus Parasite:

  • Medication: The recommended treatment includes a 5-day course of Levamisole (commercially known as Ergamisol) or a combination treatment using Fenbendazole and Paracide X or D.
  • Water Change and Cleaning: After completing the treatment, conduct a major water change, ideally more than 90% of the tank’s volume. Additionally, thoroughly vacuum the substrate to remove any remaining parasites or eggs.
  • Follow-up Treatment: To ensure complete eradication, repeat the entire treatment process after 3 weeks.

14. Fish Tuberculosis

Fish Tuberculosis is caused by bacteria of the genus Mycobacterium. Common symptoms include loss of appetite, ulcers around the anus and on the body, fading colors, and fin and tail rot.

Treatment of Fish TB:

  • Incurable and Zoonotic: Unfortunately, Fish TB is incurable and zoonotic, meaning it can be transmitted to humans. Therefore, extreme caution is advised when handling affected fish.
  • Isolation and Disposal: Promptly isolate and humanely euthanize fish suspected of having TB to prevent its spread within the aquarium. Due to its high rate of proliferation and potential to be passed to offspring, immediate action is crucial.
  • Safety Precautions: Always wear protective gloves and avoid direct contact when dealing with fish suspected of having TB. Thoroughly clean and disinfect any equipment used in the affected tank.
  • Monitoring and Prevention: Regularly monitor other fish for symptoms and maintain excellent water quality to reduce stress and vulnerability to diseases.

15. Bent Spine

Bent spine, or scoliosis, in fish is often a result of genetic factors or developmental diseases in younger fish. This condition causes a noticeable curvature of the spine, leading to difficulties in swimming and general mobility.

Mollies with scoliosis may face challenges in normal development, exhibit weakness compared to their peers, and could be more prone to bullying by other fish. Additionally, these fish often have a reduced lifespan due to the associated health complications.

Managing Bent Spine:

  • No Cure: Unfortunately, there is no cure for a bent spine, as it is typically linked to genetic factors.
  • Preventive Measures: To mitigate the spread of this condition, avoid breeding fish that exhibit signs of scoliosis. It’s important to select healthy, robust adults for breeding purposes.
  • Careful Observation: Regularly monitor your fish for any signs of health issues, including spinal deformities, to manage your aquarium’s health effectively.

16. Hexamitiasis

Hexamitiasis, caused by the Hexamita parasite, is not commonly found in mollies, but it can infect them under certain conditions.

Symptoms of Hexamitiasis:

  • Loss of coloration.
  • Lesions on the head or body.
  • Decreased appetite.
  • Difficulty in swimming.

Treatment of Hexamitiasis:

  • Medicated Food: Administer medicated food containing metronidazole (commercially known as Flagyl) to treat mollies infected with Hexamita.
  • Water Treatment: If the fish refuse to eat, treat the tank water with 250 mg of metronidazole per 10 gallons of water once daily for at least 3 days.

General Advice for Molly Fish Care:

  • It is crucial for fish keepers to familiarize themselves with the symptoms and treatment options for common diseases.
  • Preventing diseases is far more effective than treating them. Regularly monitoring your fish and maintaining good aquarium conditions are key preventive measures.
  • The following section will provide insights on how to prevent diseases, parasites, and other health issues in molly fish.

17. Red Gills

Red gills in molly fish can be a sign of various environmental stressors or health issues. This condition is noticeable by the distinct reddening of the gills, which may be accompanied by other symptoms such as rapid gill movement or labored breathing.

Common Causes of Red Gills:

  • Poor Water Quality: High levels of ammonia, nitrites, or low oxygen levels can cause irritation and redness in the gills.
  • Bacterial or Parasitic Infections: These can directly infect the gills, leading to inflammation and redness.
  • Chemical Irritants: Overmedication or exposure to harmful chemicals in the water can also lead to red gills.

Treatment and Prevention:

  • Water Quality Check: Regularly test water parameters to ensure optimal conditions. Ammonia and nitrite levels should be near zero, and oxygen levels should be high.
  • Correct Water Conditions: Perform regular water changes and ensure proper filtration to maintain a healthy environment.
  • Isolate Affected Fish: If a bacterial or parasitic infection is suspected, isolate the affected fish to prevent the spread to other tank inhabitants.
  • Consult a Specialist: For persistent problems or if the exact cause is unclear, consult a veterinarian who specializes in fish health.

Quick & Practical Molly Fish Disease Prevention Tips

Maintaining the health of your molly fish requires diligence and care. Here’s a disease prevention checklist to help keep your mollies strong and disease-free:

  1. Optimal Water Conditions: Ensure the water temperature and other parameters are consistently at required levels.
  2. Regular Tank Maintenance: Perform routine water changes and clean the tank regularly.
  3. Proper Tank Cycling: Complete the nitrogen cycle in your tank without rushing the process.
  4. Balanced Diet: Provide a varied and well-balanced diet; avoid overfeeding.
  5. Remove Waste Promptly: Clear the tank of dead fish, decaying plants, or uneaten food immediately.
  6. Monitor Health and Water Quality: Regularly check your fish for signs of disease and keep an eye on water quality.
  7. Quarantine New Additions: Isolate new fish for 3-4 weeks before introducing them to the main tank.
  8. Isolate Sick Fish: Transfer fish showing illness symptoms to a hospital tank for quarantine and treatment.
  9. Disinfect New Plants: Carefully disinfect new plants before adding them to your aquarium.
  10. Stock Basic Medications: Keep medications for common molly diseases on hand.
  11. Responsible Breeding: Avoid breeding unhealthy fish or those with genetic conditions.
  12. Avoid Overstocking: Maintain a balanced fish population to ensure adequate oxygen levels and control waste accumulation.
  13. Minimize Stress: Reduce potential stressors in the aquarium environment.
  14. Compatibility Check: Ensure that all fish in your tank are compatible.
  15. Remove Aggressive Fish: Aggressive or fin-nipping fish should be removed to prevent injuries and subsequent infections.
  16. Caution with Live Foods: Be mindful when feeding live foods, as they can carry diseases.

Prevention is easier and often more effective than treating diseases. Many fish diseases are curable, but some, like dropsy, are not. By following these preventive measures and managing your aquarium with care, you significantly reduce the risk of disease in your molly fish.

Questions and Answers

Tom Berry June 27, 2019 Reply

Thanks for all the advice, its really helpful to my 14 year old son who’s just starting on his aquarium journey …Tom.

Debbie smith November 28, 2019 Reply

One of my black mollys has gone from being really active to swimming head downwards struggling to get upright and I notice it has a white substance coming out of his anal and the anal is also white I don’t know what’s wrong and what to do for him as this has all happened in one day

    Hi! This sounds like it could be a problem with the swim bladder. If your other fish are also affected, then probably it is ammonia poisoning and you should do a 30% water change and monitor the ammonia and nitrite levels in the next hours and days. This is the most common reason, why mollies die.
    If only this fish has this problem, than you can’t do much about it, because the swim bladder is an organ which helps with the buoyancy of the fish. Swim bladder disorder can’t be treated in most cases, so you can’t do much about it.
    Make sure the water parameters are right to avoid this problem in the future.

Hi! My brother’s balloon molly has a orangy- brown pretension coming out of his side. I tried treating with aquarium salt, but it didn’t help. It almost looks like part of him is decomposing. It’s not affecting the other fish in the tank and it doesn’t seem to be affecting him very much, but his organs are close to the surface of his skin which could be bad. Any thoughts?

    This can happen due to some sort of infection. Most likely it is not treatable. If it doesn’t affect the other fish, probably it is a genetic problem with that particular molly. In any case, you should separate the sick molly from the rest.

Anahstrailya Ball February 23, 2020 Reply

Hey there, just had a Molly question. Our Molly has pop eye, is having breathing problems, is seizing up, and isn’t as active as before. We just now found what looked to be a little wriggling hair moving towards the heater. Any idea what the actual fresh hell this could possibly be??

    The cause of pop eye in fish is really hard to determine. I suggest you check water parameters first. If your tank has 0ppm ammonia and 0ppm nitrite, most likely your fish has a bacterial or fungal infection. Treating your fish with Seachem Paraguard can help treating this disease. If your other fish look healthy, than you can only treat this fish alone in a separate tank. If your other fish have sign of disease, you should treat your whole tank.

Carri Carothers October 13, 2020 Reply

My 8 yo black molly has gold blisters on his forehead. They seem to swell, then settle down to black bumps. Help plz? Thx!!!!

    This sounds like Velvet diseases. The Seachem Paraguard medication can help with this disease, but if your fish is so old, maybe no medication can help anymore.

    Sue Williams November 2, 2020 Reply

    Hi there, our gold molly has a lump protruding from its head. Its a dark colour.
    The fish is swimming and eating normally. Any ideas?

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