Molly Fish With Bulging Eyes: Treating Popeye Disease

Disclosure: When you purchase something through my affiliate links, I earn a small commission. As an Amazon Associate, I earn from qualifying purchases.

I was pretty nervous when I first saw one of my molly fish developing bulging eyes. I had no idea what was going on or what to do.

Fortunately, over the years, I learned more and more about this condition and found useful ways to deal with it and possibly reverse the symptoms.

In this article, I will walk you through what is known as Popeye disease in molly fish. You will learn how to treat it, and how to prevent it from reoccurring in the future.

Let’s get started.

What Exactly Is Popeye Disease in Molly Fish?

Popeye disease, also referred to as exophthalmia, is a common condition observed in molly fish and various other species kept in aquariums.

It leads to an abnormal bulging or protrusion of one or both eyes, caused by an accumulation of fluid behind the eyeball.

The condition, which can result from factors such as bacterial infections, injuries, or poor water conditions, causes discomfort and potentially severe complications for the affected fish.

Also Read: 15 Molly Fish Diseases & Their Treatments

What Are the Symptoms of Popeye Disease in Molly Fish?

The symptoms of Popeye disease in molly fish are usually quite evident. Being aware of these signs can be helpful in identifying and treating this disease at an early stage:

  • Eye Protrusion: The most distinctive symptom is one or both of the molly fish’s eyes bulging or protruding noticeably more than normal.
  • Cloudy Appearance: The affected eyes of the molly fish may turn cloudy, which indicates an internal issue causing the Popeye disease.
  • Inflammation Around the Eye: The skin or scales around the affected eye(s) of the molly fish might become inflamed or discolored.
  • Swimming Difficulties: Molly fish with Popeye disease may exhibit erratic or less coordinated swimming patterns due to impaired vision.
  • Loss of Appetite: Affected molly fish may show reduced interest in food or decline to eat altogether, which can lead to weight loss.
  • Lethargy: The molly fish may appear less active or lethargic, signaling overall discomfort or weakness.
  • Hiding: Molly fish suffering from Popeye disease might hide more often, indicating stress or discomfort.

What Causes Popeye Disease in Molly Fish?

The causes of Popeye disease in molly fish can be varied, involving factors such as infection, injury, or bad water conditions.

Understanding these causes is the first step towards effective prevention and treatment:

1. Infection

Infections, often bacterial in nature, are a primary cause of Popeye disease in molly fish:

  • Bacterial Presence: Bacterial infections are frequently behind Popeye disease, as they can cause inflammation and fluid accumulation, leading to the characteristic eye protrusion.
  • Secondary Symptoms: Along with the eye swelling, infected molly fish may display additional symptoms like clamped fins, raised scales, or abnormal swimming behaviors.
  • Common Culprits: Certain bacterial strains like Pseudomonas and Aeromonas are often implicated, especially in poorly maintained aquarium conditions.
  • Disease Progression: If left unchecked, the infection can worsen and may lead to severe complications like dropsy or fin rot in molly fish.

Also Read: Molly Fish Ich

2. Injury

Physical injuries can also lead to Popeye disease in molly fish, often discernible when only one eye is affected:

  • Single Eye Affectation: If Popeye disease only affects one eye of the molly fish, it can indicate a localized injury rather than a systemic issue.
  • Tank Hazards: Sharp objects, aggressive tank mates, or unsafe handling can cause physical trauma leading to the disease.
  • Immediate Swelling: After the injury, the eye of the molly fish may swell up almost immediately due to inflammation or internal bleeding.
  • Potential Infection: An injury may also open the door for secondary infections, thus exacerbating the condition in molly fish.

3. Bad Water Conditions

Poor water conditions in the aquarium can stress molly fish, compromising their immune system and making them more susceptible to Popeye disease:

  • Ineffective Filtration: When the filtration system is inadequate, harmful substances like ammonia and nitrites can accumulate, causing stress to molly fish and increasing the likelihood of developing diseases.
  • Incorrect Water Parameters: Parameters like pH, temperature, and hardness, if not suitable for molly fish, can cause stress and promote the onset of diseases.
  • Overcrowding: Overpopulated tanks can quickly deteriorate in water quality, leading to increased disease transmission among molly fish.
  • Infrequent Water Changes: Neglecting regular water changes can cause an accumulation of waste products and pathogens, creating an environment conducive to Popeye disease in molly fish.

How Is Popeye Disease Diagnosed in Aquarium Fish?

Diagnosis of Popeye disease in aquarium fish, including molly fish, primarily involves careful observation of physical symptoms, especially the characteristic eye swelling or protrusion. 

Additional signs such as changes in behavior, appetite, and general health, coupled with an assessment of the aquarium’s water quality and condition, can also aid in diagnosing this condition.

How to Treat Popeye Disease in Molly Fish

Treatment strategies for Popeye disease in molly fish depend on the underlying cause.

By addressing infections, injuries, and poor water quality, the disease can be managed effectively:

1. Managing Popeye Disease Resulting from an Infection

Treating bacterial infections that cause Popeye disease in molly fish involves targeted medication and meticulous care:

  • Medication Use: Utilize antibiotics such as tetracycline or kanamycin, commonly used in a dosage of 5-10 mg per liter of tank water. These medications are particularly effective against gram-negative bacteria often implicated in Popeye disease.
  • Commercial Products: Products available online can assist your molly fish in fighting the underlying infection. My recommendation: API MELAFIX (link to Amazon).
  • Isolation: Transfer the infected molly fish to a hospital tank. This minimizes the risk of disease spread, which can be as high as 30-40% in crowded tanks.
  • Medicated Food: If the molly fish maintains an appetite, providing medicated food enhances the effectiveness of treatment. Typically, medicated food can be given for 5-10 days, ensuring full-course antibiotic delivery.
  • Regular Water Testing: Test water parameters every 2-3 days during treatment. Maintaining a pH around 7.0-7.8, temperature 72-82°F, and low levels of ammonia and nitrites can accelerate the molly fish’s recovery.
  • Follow-up Observation: Post-treatment, monitor the molly fish for at least 2 weeks. This helps ensure complete recovery and early detection if Popeye disease reoccurs.

2. Addressing Popeye Disease Caused by an Injury

Proper care following an injury can prevent or treat Popeye disease in molly fish:

  • Isolation: Isolate the injured molly fish in a hospital tank. This reduces stress and prevents further injury or bullying.
  • Mild Salt Baths: Implementing short salt baths, typically 5-10 minutes in a 1-3% salt solution, can help reduce inflammation and prevent secondary infections.
  • Removal of Hazards: Eliminate sharp objects or aggressive tank mates from the aquarium. Around 50% of injuries can be attributed to such environmental hazards.
  • Monitoring Tankmates: Pay close attention to the behavior of other fish in the tank. Address aggressive behavior promptly, as it is a significant cause of injuries among tankmates.
  • Regular Observation: Check the injured molly fish daily to monitor healing and detect any worsening conditions. This is crucial during the first week post-injury.

3. Treating Popeye Disease Resulting from Poor Water Quality

Improving and maintaining water quality is crucial in treating Popeye disease in molly fish:

  • Water Changes: Implement regular and thorough water changes, ideally 25-50% of the tank volume weekly. This helps maintain water quality and reduces stress in molly fish.
  • Proper Filtration: Install a robust filtration system. Ensuring 4-6 times the tank volume in turnover rate per hour can effectively remove harmful substances. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Testing Water Parameters: Regularly test and adjust water parameters. Maintaining a pH of 7.5-8.2, hardness of 10-25 dGH, and temperature around 75-80°F is optimal for molly fish. My recommendation: API FRESHWATER MASTER TEST (link to Amazon).
  • Managing Tank Population: Avoid overcrowding in the tank. A general rule of thumb is to have 1 inch of fish per gallon of water to maintain a healthy environment.
  • Stress Reduction: Create a suitable habitat with sufficient hiding spots and plant life. Providing a stress-free environment can enhance the molly fish’s immune response.

What Is the Prognosis for Molly Fish with Popeye Disease?

The prognosis for molly fish with Popeye disease largely depends on the timeliness and effectiveness of the treatment.

If diagnosed and treated early, molly fish can recover fully, but delayed intervention or severe cases can lead to complications like blindness or even death.

How Can Popeye Disease in Molly Fish Be Prevented?

Preventing Popeye disease in molly fish primarily involves maintaining optimal water conditions and reducing stress factors:

  • Regular Water Changes: Carry out consistent water changes, replacing around 25-50% of the water weekly. This ensures optimal water quality and significantly reduces disease risk.
  • Effective Filtration: Use an effective filtration system, capable of turning over 4-6 times the tank volume per hour. This keeps the water conditions suitable for molly fish.
  • Appropriate Tank Conditions: Maintain water parameters within the correct range for molly fish. A pH of 7.5-8.2, hardness of 10-25 dGH, and temperature of 75-80°F are generally ideal.
  • Balanced Diet: Provide a balanced and varied diet. Around 70% plant-based food and 30% protein-based food can boost molly fish’s health and immunity.
  • Regular Observations: Monitor your molly fish regularly. Early detection of symptoms can decrease the progression rate significantly.


For those of you in a rush, here is a quick overview of what I discussed above:

  • Popeye disease in molly fish is commonly caused by infections, injuries, or poor water conditions, leading to eye protrusion and discomfort.
  • Recognizing symptoms, providing targeted treatment, and maintaining optimal water conditions are crucial for managing and preventing Popeye disease in molly fish.
  • Timely intervention and effective treatment can lead to full recovery, while delayed treatment or severe cases may result in complications such as blindness or death.
  • Regular observation, proper care, and preventive measures such as regular water changes and maintaining suitable tank conditions can significantly reduce the risk of Popeye disease in molly fish.
  • By understanding the causes, symptoms, and treatment options for Popeye disease, aquarists can ensure the well-being of their molly fish and promote a healthy aquarium environment.