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Molly Fish Constipation: Causes, Symptoms & Treatments

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Mollies that become constipated are the real deal. This situation can be overwhelming and possibly jeopardize the fish’s life.

When I first encountered this condition, I panicked. I had no idea what to do or what was going on. Fortunately, over the years, I gained some experience in this field.

In this article, you’ll learn what causes constipation in molly fish, how to treat it, and how to prevent this from happening again in the future.

Let’s get started.

What Causes Constipation in Molly Fish?

Several factors may cause constipation in mollies:

1. Insufficient Nutrition

Molly fish require a balanced diet rich in fiber to maintain a healthy digestive system. Insufficient nutrition can lead to constipation:

  • Poor Diet: Molly fish thrive on a diet rich in plant-based foods. A diet lacking in fibrous content can cause constipation.
  • Lack of Variety: A monotonous diet that doesn’t offer a variety of nutrients can lead to digestive problems in molly fish.
  • Overreliance on Dry Foods: Too much dry food, which lacks moisture content, may lead to hard stools in molly fish.

2. Overfeeding

Constipation in molly fish is often attributed to overfeeding:

  • Excessive Intake: Molly fish possess small stomachs, and overfeeding can lead to digestive tract obstructions, thereby causing constipation.
  • Irregular Feeding Schedule: Administering large quantities of food sporadically can result in overconsumption, which is harmful to the molly fish’s digestion.
  • Fat-Rich Foods: Foods laden with high levels of fat can precipitate bloating and constipation in molly fish.

3. Inadequate Physical Activity

A lack of movement can also induce constipation in molly fish:

  • Constricted Space: Housing a molly fish in a compact tank can limit its physical activity and trigger digestive complications.
  • Environmental Stimulation Deficiency: A setting that lacks stimulating features and interaction can result in diminished activity in molly fish.
  • Stress: Stress can reduce activity and has the potential to impair the digestive system of molly fish.

4. Reduced Water Temperature

Chilly water temperatures can decelerate molly fish’s metabolism, giving rise to constipation:

  • Decreased Metabolism: Cooler water temperatures can reduce the metabolic rate in molly fish, thereby slowing digestion and causing constipation.
  • Insufficient Heater: In the absence of an adequate heater, the water temperature can fall, negatively affecting the digestive health of molly fish.
  • Temperature Variability: Recurrent fluctuations in water temperature can induce stress in molly fish, disrupting their digestive process.

5. Surface-Level Feeding

Feeding your molly fish at the water’s surface can lead to them swallowing air, which can result in constipation:

  • Air Consumption: Ingesting air during feeding can trigger bloating and constipation in molly fish.
  • Fast Consumption: Surface-level feeding can make molly fish eat more hastily, resulting in indigestion and constipation.
  • Floating Food: Specific types of food that linger on the surface can promote air consumption in molly fish.

Also Read: 15 Molly Fish Diseases & Their Treatments

What Are the Symptoms of Constipation in Molly Fish?

Fortunately, a constipated molly fish is pretty easy to identify. Here are the symptoms to look out for:

1. Swollen Abdomen

A constipated molly fish might exhibit a swollen abdomen:

  • Visible Distension: The fish’s belly may appear visibly swollen due to the accumulation of feces.
  • Color Changes: There might be noticeable color changes around the abdomen of the molly fish.
  • Prolonged Swelling: A swollen abdomen that persists over time is a strong indication of constipation in molly fish.

2. Failure to Excrete/Stringy Feces

Molly fish suffering from constipation may struggle to excrete or produce stringy feces:

  • Lack of Feces: Not seeing any waste for prolonged periods could indicate constipation in molly fish.
  • Unusual Feces: Stringy or unusually formed feces can suggest constipation in molly fish.
  • Visible Straining: The fish may show signs of discomfort or straining during excretion.

3. Reduced Activity

A constipated molly fish might exhibit less enthusiasm in swimming or other activities:

  • Lethargy: The fish may seem less active or show signs of fatigue, a common symptom of constipation in molly fish.
  • Loss of Interest: The molly fish may lose interest in food or interaction, indicating discomfort.
  • Resting Frequently: If your molly fish is resting at the bottom of the tank more frequently, it might be constipated.

4. Difficulty Swimming

Molly fish with constipation can show signs of difficulty swimming:

  • Unusual Swimming Patterns: The molly fish might swim sideways, struggle to maintain balance, or have a hard time rising to the water’s surface.
  • Erratic Movement: Sudden or jerky movements can also be a sign of discomfort in molly fish due to constipation.
  • Struggling to Swim: If your molly fish appears to be working harder than usual to swim, this might indicate constipation.

5. Swim Bladder Issues

Constipation can lead to swim bladder issues in molly fish:

  • Buoyancy Problems: The molly fish may struggle to control its buoyancy, a symptom often linked with constipation.
  • Swimming at an Angle: If the molly fish is swimming at an odd angle, this could indicate a swim bladder issue caused by constipation.
  • Inflated Swim Bladder: An inflated swim bladder, seen as a swollen belly, can be a sign of constipation in molly fish.

Also Read: Molly Fish Swim Bladder Disease

How to Manage Constipation in Molly Fish

If you suspect that your molly fish is constipated, simply follow these steps:

1. Temporarily Fast Your Molly Fish

A short fast can give the molly fish’s digestive system a chance to clear any blockages:

  • Period of Fasting: A fasting period of about 24 to 48 hours can often help relieve constipation in molly fish.
  • Monitor Closely: Monitor the molly fish’s behavior and signs of improvement during this period.
  • Resume with Light Feeding: Once the fasting period is over, start with feeding small amounts of easily digestible foods.

2. Offer Peas as a Dietary Option

Peas are a natural laxative for molly fish and can help manage constipation:

  • Cooked and Shelled Peas: Feeding them shelled, boiled peas, mashed to an appropriate size can aid digestion.
  • Fiber-Rich Food: The high fiber content in peas can help alleviate constipation in molly fish.
  • Moderation is Key: While beneficial, peas should be given in moderation, as part of a varied diet.

3. Consider Using Epsom Salt

Epsom salt can be used to relieve constipation in molly fish. Here is what you should do:

  • Add to Tank: A small amount of Epsom salt, around 1-3 teaspoons per 5 gallons of water, can be added to the tank. My recommendation: Natural Epsom Salt (link to Amazon).
  • Short-Term Solution: Epsom salt is a temporary solution and should be used carefully to avoid causing additional stress to the molly fish.
  • Consult a Vet: Before using Epsom salt, it’s advised to consult with a fish veterinarian to ensure the safe treatment of constipation.

4. Increase the Water Temperature

A slightly higher water temperature can speed up the molly fish’s metabolism and alleviate constipation:

  • Raise Gradually: The temperature should be increased gradually to avoid shocking the molly fish, aiming for a range of 78-82°F.
  • Monitor Molly Fish: It’s crucial to keep an eye on the molly fish’s behavior with the temperature change to ensure it’s beneficial.
  • Use a Thermostat: A reliable aquarium heater with a thermostat will help maintain a steady temperature.

5. Perform Regular Water Changes

Clean water is essential to maintaining the health and digestive system of molly fish:

  • Weekly Changes: Changing 10-20% of the water weekly can significantly improve the living conditions for molly fish.
  • Eliminate Waste: Regular water changes help eliminate waste and uneaten food, reducing the chances of constipation.
  • Stable Environment: This practice also helps to keep the tank environment stable, decreasing stress for the molly fish.

Also Read: Black Spots On Molly Fish

What Are Other Possible Causes of a Swollen Stomach in Molly Fish?

Bear in mind that a molly with a swollen belly isn’t necessarily constipated, as there is a differential diagnosis here. Consider the following:

1. Dropsy

Dropsy is a common fish disease where fluid accumulates in the body cavities, causing a swollen belly in molly fish:

  • Infectious Disease: Dropsy is often a sign of a bacterial infection that leads to fluid buildup, making the belly of molly fish appear swollen.
  • Pinecone Scale Pattern: The scales of molly fish suffering from dropsy will protrude outwards, often described as a pinecone-like appearance.
  • General Unwellness: Molly fish with dropsy might also show signs of lethargy, loss of appetite, and faded color.

2. Pregnancy

Female molly fish have a gestation period of around 60-70 days, during which their belly will appear swollen:

  • Gravid Spot: A dark gravid spot near the anal vent is a clear indication of pregnancy in molly fish.
  • Behavioral Changes: Pregnant molly fish may show signs of hiding or nesting behavior, along with a swollen belly.
  • Increased Size Over Time: The belly of a pregnant molly fish will progressively swell over the course of the gestation period.

3. Tumors

Although less common, tumors can cause a swollen stomach in molly fish:

  • Uneven Swelling: Unlike constipation or dropsy, tumors often cause an uneven swelling in the stomach of molly fish.
  • Growth Over Time: The belly will continue to swell over time if the molly fish has a tumor.
  • Lumps or Bumps: Any lumps or bumps on the molly fish’s body should be evaluated by a veterinarian for a potential tumor.

How to Avoid Constipation in Molly Fish?

As you already know, prevention is always better than treatment. Here is how you can prevent constipation in the future:

1. Offer a Balanced Diet

Ensuring a well-rounded diet is vital for supporting a healthy digestive system in molly fish:

  • Diverse Food Items: A diverse diet, consisting of flakes, pellets, and both live and frozen foods, can help avert constipation in molly fish.
  • Foods Rich in Fiber: Adding fiber-enriched foods such as boiled peas to molly fish’s diet can boost their digestive process.
  • Controlled Feeding: Overeating can result in constipation, so it’s important to feed your molly fish in regulated amounts.

2. Guarantee Adequate Space in the Aquarium

Molly fish require sufficient swimming room to uphold good digestive health:

  • Suitable Aquarium Size: For molly fish, it’s recommended to have a tank size of at least 20 gallons to provide enough space for swimming.
  • Controlled Fish Population: Overpopulation can stress the fish and cause health problems, including constipation.
  • Engaging Habitat: Offering an engaging environment with plants and hiding places encourages molly fish to remain active.

3. Maintaining Optimal Water Conditions

Ensuring the right water temperature is pivotal to promoting a healthy metabolism in molly fish:

  • Trustworthy Heater: Employing a reliable heater is key to sustaining the optimal temperature for your molly fish, typically between 72-82°F. My recommendation: Orlushy Submersible Aquarium Heater (link to Amazon).
  • Steady Temperature: Steer clear of frequent temperature changes as they can trigger stress in molly fish, and strive for a stable temperature.
  • Frequent Checks: Regularly monitor and modify the water temperature to confirm it’s within the ideal range for molly fish.

Can Constipation Prove Fatal for Molly Fish?

Yes, constipation can turn out to be fatal for molly fish if left untreated.

It can lead to extended discomfort, malnutrition, and other health complications such as swim bladder disease, considerably shortening the fish’s life expectancy.

Is It Okay to Give Molly Fish Canned Peas?

Canned peas can be served to molly fish, though it’s essential they do not contain any salt. 

Moreover, the outer skin should be peeled off, and the pea crushed to a size suitable for the molly fish.

That said, fresh peas are a preferred option due to their lack of added preservatives or sodium.


If you are in a rush, here is a brief summary of what I discussed above:

  • Molly fish constipation can be caused by insufficient nutrition, overfeeding, inadequate physical activity, reduced water temperature, and surface-level feeding.
  • Symptoms of constipation in molly fish include a swollen abdomen, failure to excrete or stringy feces, reduced activity, difficulty swimming, and swim bladder issues.
  • Managing constipation in molly fish involves fasting, offering peas as a dietary option, using Epsom salt, increasing water temperature, and performing regular water changes.
  • A swollen stomach in molly fish can also be caused by dropsy, pregnancy, or tumors, which require differential diagnosis.
  • To prevent constipation in molly fish, provide a balanced diet, ensure adequate space in the aquarium, maintain optimal water conditions, and be aware that constipation can be fatal if left untreated.