Why Do My Mollies Keep Dying? (With 7 Solutions)

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If your molly fish keep dying in your tank no matter what you do – you have come to the right place.

As this happened to me in the past, I realize how frustrating it can be. Fortunately, over the years, I learned how to deal what that issue.

In this article, I will show you what to do if your mollies keep dying over and over again in your tank. You will also learn how to deal with a molly that is dying but still alive.

So, without further ado, let’s get started.

Why Are My Molly Fish Keep Dying?

If your molly fish keep dying over and over again, there are a few things that may be causing it:

1. Inadequate Water Conditions

Molly fish thrive in clean water conditions with a pH range of 7.5 to 8.5 and a moderate level of salinity.

Unfavorable water conditions can lead to increased stress and susceptibility to diseases in mollies. Consider the following:

  • Sub-optimal pH levels: Molly fish are susceptible to rapid shifts in pH. Maintaining a stable pH level is crucial for their well-being and longevity.
  • Insufficient Salinity: Although they can survive in freshwater, mollies prefer brackish conditions. Lack of sufficient salt can negatively affect their health.
  • High Ammonia/Nitrate Levels: Inadequate filtration can lead to a buildup of toxic substances like ammonia or nitrates, posing a threat to molly fish.

2. Excessive Feeding

Overfeeding mollies can disrupt their digestion and even lead to their premature death. It can also contaminate the tank water, exacerbating the issue. Here is what you should know:

  • Poor Digestive Health: Mollies have a small stomach and they digest food quickly. Overfeeding can lead to bloating and constipation.
  • Water Contamination: Excess food that mollies cannot consume decomposes in the tank, elevating toxicity levels.
  • Obesity-Related Issues: Overeating can lead to obesity in mollies, predisposing them to various health conditions.

3. Stress Factors

Molly fish are highly sensitive to stressful conditions, which can significantly reduce their lifespan.

Major stressors include poor water conditions, aggressive tank mates, or overcrowded tanks. Keep in mind the following:

  • Unfavorable Water Conditions: Uncontrolled parameters such as temperature fluctuations, pH swings, or high toxin levels can induce stress in mollies.
  • Aggression from Tank Mates: Mollies can be stressed by aggressive or incompatible species. Choose tank mates carefully for a harmonious environment.
  • Overcrowding: Too many fish in a small tank can result in competition for resources and increased stress. Ensure there is adequate space for each molly.

4. New Tank Syndrome

Mollies can experience stress when adjusting to a new tank environment. This stress could potentially be harmful, leading to ‘New Tank Syndrome.’ Here’s what to keep in mind:

  • Shock from New Environment: Molly fish need time to adjust to a new tank. Rapid changes in environment can lead to stress, often causing illness or death.
  • Inadequate Tank Cycling: New tanks may not have cycled adequately, resulting in harmful levels of ammonia and nitrite which can be fatal to mollies.
  • Mismatched Water Parameters: If the water parameters of the new tank don’t match with those of their previous environment, mollies can become stressed and more susceptible to diseases.

5. Diseases and Health Issues

Mollies are susceptible to various diseases and health issues, which can lead to their death if not treated in time. Understanding the following points is crucial:

  • Common Illnesses: Mollies can suffer from common fish diseases such as Ich, Velvet, or fin rot. Early detection and appropriate treatment are key to ensuring their survival.
  • Parasitic Infections: Parasites like anchor worms or flukes can harm molly fish, leading to poor health or death if left untreated.
  • Genetic Disorders: Some mollies might have genetic disorders that reduce their lifespan. These disorders can often be hard to identify and address.

6. Sudden Temperature Fluctuations

Molly fish prefer consistent temperatures between 72 and 78 degrees Fahrenheit. Rapid temperature changes can cause stress and increase susceptibility to diseases.

Here is what you should know:

  • Thermal Shock: Sudden rises or drops in temperature can cause thermal shock in mollies, leading to rapid stress or even death.
  • Increased Disease Susceptibility: Stress from temperature fluctuations can lower the immune system of molly fish, making them more prone to diseases.
  • Disrupted Metabolic Functions: Mollies’ metabolic rate is dependent on the temperature of their environment. Rapid temperature changes can disrupt these processes, affecting their overall health.

7. Abrupt Water Alterations

Abrupt changes in water conditions, like a sudden change in pH or hardness, can prove fatal for molly fish. It’s important to keep the following in mind:

  • Rapid pH Changes: Mollies need a stable pH for their survival. Rapid fluctuations in pH can stress them, leading to potential health issues.
  • Shifts in Water Hardness: Molly fish prefer moderately hard water. Sudden changes in water hardness can upset their biological functions and potentially cause death.
  • Abrupt Salinity Alterations: As brackish water fish, mollies prefer some level of salinity. An abrupt change in salinity can stress mollies, affecting their health and lifespan.

Also Read: 15 Things You Should Know About Molly Fish

What To Do If Your Mollies Keep Dying

Here are a few steps you can take if your mollies keep dying in your tank:

1. Addressing Poor Water Conditions

Ensuring optimal water conditions is critical for the health and longevity of molly fish. This includes monitoring and managing the pH, salinity, and ammonia/nitrate levels in the water:

  • Regular pH Testing: Use a reliable test kit to check pH regularly, ideally once every week. Adjust it within the 7.5 to 8.5 range suitable for molly fish using pH buffers. I personally use the API FRESHWATER MASTER TEST (link to Amazon).
  • Add Aquarium Salt: Consider adding 1-2 teaspoons of aquarium salt per gallon of water to mimic the brackish conditions mollies prefer, but consult with an expert for the right quantities.
  • Effective Filtration: Ensure your tank has an effective filtration system. Aim for a filter that can handle at least twice the volume of your tank to keep ammonia and nitrate levels low. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Routine Water Changes: Perform regular water changes, approximately 25-30% of the total volume per week, to maintain clean conditions.
  • Ammonia/Nitrate Monitoring: Use testing kits to monitor ammonia and nitrate levels frequently. The ideal level for both is 0 ppm, anything higher can stress or harm molly fish.

2. Controlling Feeding Habits

Giving mollies the right amount and type of food can help stave off obesity and keep their water clean. Here are some simple, effective measures to ensure they’re eating well:

  • Feeding Schedule: It’s best to feed your mollies 2-3 times daily, making sure they finish all the food within 2-3 minutes.
  • Proportionate Portion Sizes: By providing just enough food that the mollies can eat in the allotted feeding time, you’ll avoid overfeeding.
  • Diverse Diet: Aim to give them a balanced diet comprising high-quality flake food, freeze-dried foods, and the occasional indulgence of fresh or frozen brine shrimp or bloodworms.
  • Monitoring Mealtime: Keep a close watch on your mollies’ eating patterns. If you notice sudden changes, it could signal potential health issues or stress.
  • Clearing Leftovers: Make it a habit to remove any remaining food after feeding. This will prevent the food from decaying and dirtying the water.

Also Read: How To Feed Molly Fish

3. Reducing Stress Levels

Keeping stress levels low is critical for your mollies’ wellbeing. Providing the right environment and companionship can do wonders:

  • Suitable Tank Friends: Mollies get along best with non-aggressive species. Guppies, platies, and other mollies are great choices for companions.
  • Spacious Tank: To avoid overcrowding, your mollies need at least a 20-gallon tank, with an extra 3 gallons for each additional molly.
  • Comforting Decor: Plants and decorations in the tank can act as cozy hideaways, making your mollies feel safe and less stressed.
  • Consistent Water Quality: Maintaining stable water conditions is key. Dramatic shifts in temperature, pH, or hardness can unsettle your mollies.
  • Regular Check-ins: Keep an eye on your mollies’ behavior. If you spot any changes in their activity, eating patterns, or visible signs of stress, take action quickly.

Also Read: Stress In Molly Fish

4. Managing New Tank Setup

Introducing mollies to a new tank should be done carefully to avoid New Tank Syndrome. A well-cycled and tested tank environment is necessary. Here’s what to do:

  • Proper Tank Cycling: Ensure the tank is well-cycled, which typically takes 4-6 weeks, before introducing molly fish. This allows beneficial bacteria to grow and control harmful ammonia and nitrites.
  • Gradual Acclimatization: Use the drip method or float the fish in the tank in their bag for about 15-30 minutes to slowly acclimate mollies to the new water parameters.
  • Matched Water Parameters: Make sure the water parameters (pH, temperature, salinity) of the new tank closely match those of the molly fish’s previous environment.
  • Initial Stocking Density: Begin with a small number of fish. Rapidly increasing the population can overwhelm the new tank’s biological filtration capacity, leading to harmful water conditions.
  • Regular Testing: Regularly test the water in the new tank for pH, ammonia, nitrites, and nitrates to ensure it remains safe for molly fish.

Also Read: Molly Fish Tank Setup

5. Prevention and Treatment of Diseases

Proactively preventing diseases and promptly treating any health issues can greatly improve the mollies’ survival rate. Here are some practical steps:

  • Quarantine New Fish: Always quarantine new fish for about two weeks before adding them to the main tank to prevent the spread of diseases.
  • Regular Health Checks: Conduct regular health checks, looking for signs of common diseases such as Ich, Velvet, or fin rot. Early detection is key to effective treatment.
  • Medication: Use appropriate medication to treat detected diseases, following the manufacturer’s instructions closely. Overuse can harm mollies or disrupt the tank’s biological balance.
  • Healthy Diet: Provide a nutritious diet to boost the mollies’ immune system and make them more resistant to diseases.
  • Good Water Quality: Maintain optimal water conditions. Many diseases are less likely to occur in clean, well-balanced water.

Also Read: 15 Molly Fish Diseases & Their Treatments

6. Maintaining Stable Water Temperature

Keeping the water temperature stable within the preferred range of molly fish (72-78°F) is crucial:

  • High-Quality Heater: Invest in a high-quality aquarium heater with a thermostat to maintain a stable temperature. My recommendation: Orlushy Submersible Aquarium Heater (link to Amazon).
  • Regular Temperature Checks: Check the water temperature daily using an accurate aquarium thermometer.
  • Heater Capacity: Ensure the heater is of the correct wattage for your tank size. Typically, 3-5 watts per gallon of water is a good rule of thumb.
  • Avoid Sudden Changes: Avoid placing the aquarium in locations subject to sudden temperature changes, like near air conditioners, heaters, or windows.
  • Backup Plan: Have a contingency plan, such as a backup heater or generator, in case of power failures or heater malfunctions.

Also Read: Molly Fish Temperature

7. Handling Sudden Water Changes

Effectively managing abrupt water changes can significantly improve molly fish survival. This includes changes in pH, hardness, and salinity. Here’s what to do:

  • Gradual Changes: Make any necessary changes to water parameters gradually over several days to avoid shocking mollies.
  • Regular Monitoring: Regularly monitor pH, hardness, and salinity using reliable test kits.
  • Buffer Solutions: Use commercial buffer solutions to adjust pH or hardness levels gradually.
  • Slow Water Replacement: When changing water, ensure new water matches the temperature and pH of the existing tank water. Replace only a portion of the water at a time, around 25-30%.
  • Pre-conditioned Water: Use pre-conditioned or aged water for replacements to ensure it is safe and stable for mollies.

Do Molly Fish Die Easily?

No, molly fish do not die easily if kept under the right conditions. They are generally hardy fish that can live up to five years with proper care.

However, like any other fish, they are susceptible to diseases and stress, especially when their environmental needs are not met.

Also Read: How Long Do Molly Fish Live?

Should I Change The Water After A Molly Fish Dies?

Yes, you should change the water after a molly fish dies.

The death of a fish can indicate a problem with the tank conditions, and it can also lead to a rise in ammonia levels as the body decomposes.

Performing a partial water change of about 25% to 30% and testing the water parameters can help to ensure the remaining fish are safe.

Also Read: Why Did My Molly Fish Disappear?

How To Tell If A Molly Fish Is Dying

To tell if a molly fish is dying, observe changes in its behavior, physical appearance, or eating habits, as these can all be signs of illness or stress. Here are some indicators:

  • Lethargy: A molly fish that’s less active than usual or hiding more often can indicate illness.
  • Changes in Appearance: Signs like pale color, spots, patches, fin rot, or bloated belly in molly fish often point to health issues.
  • Loss of Appetite: If your molly fish is eating less or not at all, it could be a sign of illness.
  • Erratic Swimming: If a molly fish is swimming in an erratic manner, such as floating on its side or upside down, it could be near death.
  • Labored Breathing: Rapid or labored breathing can indicate that the molly fish is stressed or ill, especially if the water conditions are not optimal.

How Do You Save A Dying Molly Fish

To save a dying molly fish, it is essential to swiftly identify the cause of the issue, then take immediate and appropriate actions to address it.

Actions can range from adjusting water conditions, isolating the affected fish, to medicating for specific diseases:

  • Immediate Isolation: If a molly fish is showing signs of illness, immediately isolate it in a quarantine tank. The quarantine tank should be cycled and ready for use, with the same water parameters as the main tank.
  • Water Parameters Check: Perform an immediate water test using a reliable test kit. The ideal parameters for molly fish include a pH level of 7.5-8.5, temperature between 72-78°F, and ammonia, nitrate, and nitrite levels as close to 0 ppm.
  • Professional Advice: If you can’t identify the issue, consider seeking help from a vet who specializes in fish, or from an experienced aquarist. They can provide advice based on specific symptoms or even conduct lab tests on a water sample.
  • Disease Identification: Look for signs of common fish diseases. For example, white spots could indicate Ich, frayed fins might signal fin rot, and rapid breathing can suggest gill disease. Each of these requires a specific type of treatment.
  • Treatment Application: Apply the necessary treatment as per the disease diagnosed. This could be a specific medication like Malachite Green for Ich or an antibiotic for bacterial infections.
  • Dietary Adjustment: If the molly fish is suffering from a dietary issue like constipation, adjust its diet. Introduce fiber-rich foods like blanched peas and reduce the amount of protein-rich food.
  • Stress Minimization: If the issue is stress-related, examine the causes. It could be aggressive tank mates, overcrowding (remember, molly fish need at least 3 gallons of water per fish), or inappropriate water conditions.
  • Long-Term Care: Following recovery, continue monitoring the molly fish closely for a few weeks to ensure there’s no relapse. Keep up with regular water changes (20-30% per week), provide a balanced diet, and ensure appropriate tank mates and conditions to prevent future issues.


For those of you in a rush, here a brief overview of what I discussed earlier:

  • The main causes of mollies dying are inadequate water conditions, excessive feeding, stress factors, new tank syndrome, and diseases/health issues.
  • Maintaining stable water conditions, proper feeding habits, and reducing stress factors can help improve the survival rate of molly fish.
  • Regular water testing, quarantining new fish, and promptly treating diseases are essential practices for preventing and addressing health issues.
  • When a molly fish is dying, immediate isolation, water parameter checks, and appropriate treatment are necessary to try and save it.
  • Proper care and attention can significantly extend the lifespan of molly fish, which can live up to five years when provided with the right environment and care.