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Molly Fish Fin Rot: Causes, Symptoms & Prevention

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A few months back, I noticed that my molly’s fins and tail looked odd. It seemed like some parts were missing, while what was left didn’t look healthy.

After extensive research, I came to an understanding that my molly was suffering from fin rot, which sometimes involves the tail as well.

In this article, I will explain everything there is to know about this condition, from diagnosing to treating it and preventing it from happening again in the future.

Let’s get started.

What Is Fin Rot in Molly Fish?

Fin rot in molly fish is a common bacterial infection that affects their fins and tails.

It is characterized by the deterioration of the fin tissue, leading to fraying, discoloration, and eventually, the loss of the affected fin.

Also Read: 15 Molly Fish Diseases & Their Treatments

What Causes Fin Rot in Molly Fish?

When we examine the reasons behind fin rot in mollies, we encounter several factors, encompassing:

1. Fin Damage

Molly fish are susceptible to fin rot if they endure damage to their fins. These injuries often originate from aquarium clashes or accidental abrasions.

Let’s explore this:

  • Conflict Outcome: Fin damage in molly fish might occur due to hostile aquarium companions, leading to fin rot.
  • Decoration Hazards: Sharp or coarse aquarium decorations can unintentionally wound molly fish fins, posing an infection risk.
  • Immunity Impairment: Damaged fins may compromise a molly fish’s overall defense mechanism, enhancing susceptibility to fin rot.
  • Infection Acceleration: If fin damages are not addressed promptly, they can hasten the progression of fin rot in molly fish.

2. Poor Water Conditions

Molly fish living in habitats with subpar water quality are at a higher risk of fin rot, as detrimental bacteria thrive in such settings.

Here’s the reason:

  • Toxic Concentrations: Elevated amounts of ammonia or nitrite in the water can harm the molly fish’s fins, facilitating fin rot.
  • Bacterial Habitat: Insufficient water hygiene can promote bacteria multiplication, escalating the risk of fin rot in molly fish.
  • Reduced Oxygen Levels: Substandard water quality often corresponds with diminished oxygen levels, inducing stress and potentially causing fin rot in molly fish.
  • Maintenance Neglect: Overlooking routine tank upkeep can progressively degrade water quality, elevating the likelihood of fin rot outbreak among molly fish.

3. Shifting Water Habitats

Sudden modifications in the molly fish’s environment can induce stress and weaken their immune system, thereby increasing their susceptibility to fin rot.

This can be broken down as follows:

  • Temperature Changes: Unanticipated fluctuations in water temperature can disorient molly fish, indirectly causing fin rot.
  • pH Variations: Abrupt alterations in water pH can injure the molly fish’s skin and fins, setting the stage for fin rot.
  • Salinity Shifts: Instant changes in water salinity can also instigate stress and debilitate molly fish, making them prone to fin rot.
  • Acclimation: Improper acclimation of newly added molly fish can trigger drastic environmental changes, adding to potential fin rot.

4. Stress

Just like other organisms, stress can weaken a molly fish’s immune system, thereby increasing their chances of fin rot.

Here’s how stress comes into play:

  • Environmental Stressors: Sudden alterations in illumination, noise, or aquarium position can unsettle molly fish, heightening the risk of fin rot.
  • Social Strains: Excessively aggressive or incompatible aquarium mates can generate stress, resulting in an increased likelihood of fin rot in molly fish.
  • Breeding Stress: The process of breeding can be a taxing event for molly fish, and this strain can possibly lead to fin rot.
  • Handling Pressure: Molly fish can undergo significant stress if they are handled too frequently or inappropriately, which can heighten the risk of fin rot.

5. Overpopulation

Keeping an excessive amount of fish in one aquarium can instigate numerous issues, including fin rot in molly fish.

Here’s how overpopulation plays a role:

  • Resource Scarcity: Overpopulation can incite competition for food and room, straining molly fish and amplifying fin rot chances.
  • Waste Buildup: A larger fish population leads to increased waste, which can lower water quality and encourage fin rot in molly fish.
  • Disease Transmission: Crowded settings can speed up the transmission of diseases like fin rot among molly fish.
  • Aquarium Capacities: Maintaining a high population of molly fish in a confined aquarium can induce stress, indirectly escalating the chances of fin rot.

Recognizing Fin Rot in Molly Fish

Diagnosing fin rot in mollies involves identifying the particular stage of the infection. Here is what you should know:

1. Mild Fin Rot

Mild fin rot in molly fish is often the first stage of the disease and if caught early, it can be treated with good success. Here’s what to look out for:

  • Minor Changes: Slight changes in the molly fish’s fin edges, such as fraying or small tears, may suggest early-stage fin rot.
  • Color Alteration: Molly fish with mild fin rot might exhibit a slight discoloration, usually white or milky, at the fin edges.
  • Behavioral Shifts: Molly fish might show subtle changes in behavior like reduced activity or appetite in early fin rot stages.
  • Swift Response: Mild fin rot in molly fish can often be reversed with prompt improvements in water quality and care.

2. Advanced Fin Rot

As the disease progresses, signs of fin rot in molly fish become more evident:

  • Significant Fin Damage: In advanced fin rot, molly fish’s fins will have more pronounced damage such as noticeable thinning or missing segments.
  • Intense Discoloration: The edges of the molly fish’s fins may turn distinctly darker or redder, indicating progressing fin rot.
  • Behavioral Changes: Molly fish may exhibit significant changes like lethargy or refusal to eat as fin rot advances.
  • Treatment Need: Advanced fin rot usually requires the use of medicated treatments alongside improvements in the molly fish’s environment.

3. Severe Fin Rot

In severe cases, fin rot can significantly compromise the health of molly fish. This stage has characteristic features:

  • Drastic Fin Loss: Severe fin rot in molly fish is characterized by substantial fin loss, often leaving only the base of the fins.
  • Body Lesions: In severe cases, the disease may spread beyond the fins, causing ulcers or lesions on the molly fish’s body.
  • Behavioral Distress: Molly fish with severe fin rot may display clear signs of distress, such as rapid breathing or erratic swimming.
  • Urgent Intervention: Severe fin rot necessitates immediate and aggressive treatment to save the affected molly fish.

4. Fin Rot Caused By Fungi

Sometimes, fungal infections can cause symptoms similar to fin rot in molly fish. Here’s what you should look out for:

  • Fungal Appearance: Fungal fin rot often presents as cotton-like growths on the molly fish’s fins, unlike bacterial fin rot.
  • Unusual Discoloration: The affected fin areas may turn an unusual color, typically a fluffy white or gray, indicating fungal infection in molly fish.
  • Slower Progression: Fungal fin rot in molly fish typically progresses slower than bacterial fin rot but still requires prompt treatment.
  • Specific Treatment: Fungal fin rot requires specific antifungal treatments to effectively combat the disease in molly fish.

Also Read: Molly Fish Ich

Approaches To Treating Fin Rot In Molly Fish

The treatment process of fin rot depends on its severity. Consider the following:

1. Treating Mild Fin Rot

If your molly fish are in the early stages of fin rot, improving their environment is key. Let’s outline this process:

  • Water Change: Replace around 50% of the tank water to quickly improve water quality, beneficial for the molly fish.
  • Thorough Tank Cleaning: Ensure to clean the tank, particularly focusing on removing any decaying organic matter such as uneaten food or plant matter.
  • Consistent Water Checks: Test water parameters daily, aiming for 0 ppm ammonia and nitrites, less than 20 ppm nitrates, a pH between 7.0-8.0, and a temperature around 75-80°F for molly fish. My recommendation: API FRESHWATER MASTER TEST (link to Amazon).
  • Quarantine Measures: Consider moving the molly fish with mild fin rot to a quarantine tank, preventing disease spread and offering a controlled treatment environment.
  • Close Observation: Observe the molly fish for a few days, ensuring fins are regrowing and the discoloration is fading. If no improvement is observed, consider medicinal treatment.

2. Managing Advanced Fin Rot

Advanced stages of fin rot in molly fish require more targeted interventions. Here are the necessary steps:

  • Aggressive Water Changes: Perform daily water changes of about 50-75% to provide the cleanest environment possible for the molly fish.
  • Antibacterial Treatment: Administer an antibacterial medication suitable for molly fish, like API MELAFIX (link to Amazon), as directed on the package.
  • Water Parameter Monitoring: Continue to monitor water parameters, ensuring they remain within a healthy range for molly fish.
  • Isolation: Isolate the affected molly fish in a separate treatment tank if you haven’t already, to avoid the disease spreading.
  • Health Evaluation: Monitor the molly fish for visible signs of improvement. If there’s no change or the condition worsens, consult a vet specializing in aquatic animals.

3. Dealing With Severe Fin Rot

Severe fin rot in molly fish requires immediate and aggressive treatment. Here’s the plan of action:

  • Veterinary Intervention: Consult an aquatic veterinarian for a proper diagnosis and treatment plan for the molly fish.
  • Intensive Water Changes: Change up to 75-90% of the tank water daily to ensure the cleanest environment for the molly fish.
  • Targeted Medication: The vet may prescribe a potent medication like Kanamycin or Tetracycline for the molly fish. Administer as per vet’s directions.
  • Strict Isolation: Maintain the affected molly fish in a hospital tank throughout the treatment to prevent cross-contamination.
  • Close Monitoring: Keep a close watch on the molly fish’s condition. Follow up with the vet regularly and be prepared to adjust the treatment plan as needed.

4. Handling Fungal Fin Rot

Fungal fin rot in molly fish requires a slightly different approach. Here’s a step-by-step guide:

  • Identification: Confirm the fluffy, cotton-like appearance on molly fish fins, a typical sign of fungal infection.
  • Antifungal Medication: Begin treatment with an antifungal medication like API Fungus Cure or Pimafix, following the package instructions.
  • Daily Water Changes: Replace 50% of the tank water daily to maintain a high-quality environment for the molly fish.
  • Quarantine Measures: Move the affected molly fish to a separate tank for treatment to avoid spreading the fungus to other fish.
  • Progress Tracking: Observe the molly fish daily for signs of improvement. If the condition doesn’t improve or worsens, seek veterinary advice.

Also Read: Black Spots On Molly Fish

How To Prevent Fin Rot in Molly Fish

Preventing fin rot in molly fish primarily involves maintaining an optimal living environment and regularly monitoring the fish’s health.

These actions reduce stress and deter harmful bacteria growth. Here’s how to ensure this:

  • Proper Tank Maintenance: Regularly clean the tank and perform partial water changes (25-30%) every week to maintain high water quality for your molly fish.
  • Monitor Water Parameters: Keep a close eye on water parameters. Molly fish thrive in a pH range of 7.0-8.0, water temperature between 75-80°F, and ammonia and nitrite of 0 ppm.
  • Adequate Space: Ensure your molly fish have enough space. A general rule of thumb is 1 gallon of water per inch of fish, but molly fish prefer spacious tanks.
  • Balanced Diet: Feed your molly fish a balanced diet of high-quality flake food and occasional fresh or frozen foods to boost their immune system.
  • Careful Tankmate Selection: Choose compatible, non-aggressive tank mates to reduce stress and potential fin damage in your molly fish, thereby reducing the chance of fin rot.


For those of you in a rush, here is a quick summary of what I discussed earlier:

  • Fin rot in molly fish can be caused by fin injuries resulting from tank confrontations or sharp tank decorations. Untreated injuries can expedite the spread of fin rot among molly fish.
  • Poor water quality, characterized by high levels of ammonia or nitrite, inadequate cleanliness, and low oxygen levels, makes molly fish more susceptible to fin rot.
  • Abrupt changes in water temperature, pH, and salinity can stress molly fish and weaken their immune system, increasing their vulnerability to fin rot.
  • Stress, caused by environmental triggers, social tensions, breeding, or incorrect handling, can undermine the molly fish’s immune system and contribute to the development of fin rot.
  • Overcrowding in a tank can lead to limited resources, waste accumulation, and the spread of diseases, including fin rot, among molly fish. Providing adequate space is crucial in preventing fin rot.