Black Spots On Molly Fish: Causes & How To Treat Them

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I really enjoy growing molly fish, mainly because they are so beautiful and bring life and energy to home aquariums.

However, sometimes they show signs that worry me. A few months ago, I noticed one of my mollies had developed what looked like black spots on its gills.

Today, I will show you what can cause this condition, how to treat it, and how to prevent it from reoccurring in the future.

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

What Causes Black Spots to Appear on Molly Fish?

If you have noticed several black spots on your molly fish, there are a few things that might have caused them:

1. Elevated Ammonia Levels

High concentrations of ammonia in aquarium water can stress molly fish, leading to discoloration or black spots:

  • Ammonia toxicity: Prolonged exposure to ammonia harms molly fish’s health, inducing the formation of black spots.
  • Inadequate filtration: Without proper filtration, ammonia levels rise, which increases the likelihood of black spots appearing on molly fish.
  • Overstocking: Keeping too many molly fish in a single aquarium boosts ammonia production, stressing fish and prompting black spots.
  • Overfeeding: Unused food decomposes and raises ammonia levels, thus contributing to the black spots problem on molly fish.

2. Black Spot Disease

This parasitic condition, alternatively known as diplopstomiasis, results in the formation of black cysts or spots on the skin of molly fish:

  • Parasite-induced infestation: The primary culprit behind black spot disease in molly fish is the Diplostomum parasite.
  • Contaminated water sources: Rapid spread of this disease occurs in tainted water, rendering other molly fish prone to black spots.
  • Intermediary hosts: Avian species and snails can act as vectors for the parasite, bringing it into contact with molly fish and precipitating black spots.
  • Compromised immunity: Molly fish with weakened immune defenses are at a higher risk of contracting black spot disease.

Also Read: 15 Molly Fish Diseases & Their Treatments

3. Healing From An Injury

Injuries can lead to the emergence of black spots on molly fish as part of the recovery or scar tissue formation process:

  • Physical damage: Wounds or abrasions in molly fish have the potential to evolve into black spots during their recuperation phase.
  • Subpar tank environment: Sharp or abrasive objects within the tank pose a risk of causing injury to molly fish, and subsequent black spots.
  • Tankmate aggression: Interactions involving aggression with other fish can create wounds that morph into black spots on molly fish.
  • Inappropriate handling: Poorly handled molly fish are prone to injuries, which could manifest as black spots during healing.

4. Genetic Factors

Certain molly fish are genetically inclined to develop black spots, which can surface irrespective of disease presence or water quality:

  • Inherited characteristics: Black spots may be a typical color deviation in some breeds of molly fish, attributable to their genetic composition.
  • Ageing: Black spots can naturally form on some ageing molly fish, pointing towards a genetic factor.
  • Melanistic gene presence: Molly fish possessing a melanistic gene may gradually develop black spots.
  • Unexpected genetic mutation: Uncommon genetic variations can trigger spontaneous development of black spots in molly fish.

Methods For Treating Molly Fish With Black Spots

Here is what you can do to deal with black spots on your molly fish:

1. Addressing Ammonia Poisoning

Ammonia toxicity can be addressed by improving water conditions and reducing stressors for the molly fish:

  • Water changes: Changing 25-50% of the tank water weekly can significantly reduce ammonia levels, improving the health of molly fish.
  • Improved filtration: Using a filter rated for a tank at least 10-20% larger than yours can more effectively break down and remove ammonia, reducing black spots in molly fish. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Appropriate stocking: Limiting your tank to one molly fish per 10 gallons of water helps manage ammonia production.
  • Proper feeding: Feeding molly fish only what they can consume in 2-3 minutes twice daily prevents overfeeding, which can cause excess food to decompose and raise ammonia levels.
  • Ammonia test kits: Regularly test the water with an ammonia test kit, ideally maintaining levels close to 0 ppm. My recommendation: API FRESHWATER MASTER TEST (link to Amazon).

2. Strategies To Control Black Spot Disease

The management of black spot disease can be achieved via specific treatment methods and by enhancing the overall conditions of the tank:

  • Antiparasitic remedies: The use of drugs or commercial products can aid in ridding molly fish of black spot disease. My recommendation: Hikari Prazipro (link to Amazon). Make sure to follow the manufacturer’s instructions.
  • Isolation: Isolating affected molly fish minimizes the risk of the disease’s transmission since, if not controlled, it can impact 70-80% of the tank mates.
  • Tank temperature: Raising the water temperature in the tank to roughly 82°F (28°C) for several days accelerates the parasites’ life cycle, enhancing the effectiveness of the treatments.
  • Eliminating secondary hosts: Snails can harbor black spot disease. Their removal can potentially diminish the disease’s prevalence by up to 90%.
  • Boosting fish immunity: Supplying molly fish with a well-balanced diet and maintaining clean water conditions can augment their immunity, making them less prone to black spot disease.

3. Encouraging Wound Recovery

Appropriate care and tank upkeep can foster wound healing and hinder the appearance of black spots following an injury:

  • Secure environment: The removal of sharp items from the tank can prevent nearly 80% of physical injuries in molly fish, thereby decreasing the emergence of black spots.
  • Diminishing aggression: Isolating aggressive fish can reduce the risk of injuries in molly fish by half.
  • Medicinal baths: Baths containing medications like malachite green can expedite the healing of injuries in molly fish.
  • Careful handling: Employing a soft mesh net to move molly fish can lower handling injuries significantly.
  • Stress Coat supplements: Utilizing stress coat supplements can help shield the skin and scales of molly fish. My recommendation: API Stress Coat (link to Amazon).

Also Read: Molly Fish Fin Rot

4. Genetic Factors To Consider

If the black spots are inherited, it’s crucial to provide proper general care and keep an eye on the molly fish for any signs of discomfort:

  • Genetic comprehension: Understanding that 5-10% of molly fish may naturally display black spots due to their genetic composition can alleviate needless concerns.
  • Routine monitoring: Frequent checks on your molly fish can help you identify any abrupt alterations in their behavior or looks, signaling potential health issues.
  • Balanced nutrition: Feeding molly fish a diverse diet can help ensure they remain healthy, even if they have genetic black spots.
  • Ideal tank conditions: Keeping the tank conditions optimal with a temperature of 72-78°F (22-26°C) and pH of 7.5-8.5 can substantially enhance the overall wellbeing of molly fish.
  • Selective breeding: When breeding molly fish, sidestep those with undesirable traits such as excessive black spots.

Also Read: Molly Fish Constipation

Prevention Tactics For Black Spots On Molly Fish

The prevention of black spots on molly fish primarily hinges on maintaining excellent water quality, ensuring a safe tank environment, providing a balanced diet, and conducting routine health assessments.

Here’s the breakdown:

  • Consistent water changes: Weekly replacement of 25% of the tank water sustains the ideal water quality for molly fish, diminishing the likelihood of black spots resulting from ammonia surges.
  • Proper stocking: Allowing ten gallons of water per molly fish helps avoid overpopulation, thereby lowering stress levels and the risk of black spots.
  • Nutritious diet: Offering molly fish a diverse diet that comprises flakes, vegetables, and sporadic treats of brine shrimp or bloodworms ensures their optimal wellbeing, making them less prone to diseases that cause black spots.
  • Secure tank environment: A tank devoid of sharp objects or aggressive companions can prevent physical injuries that may lead to black spots in molly fish.
  • Efficient filtration: Utilizing a filtration system designed for a tank 10-20% larger than yours can help uphold water quality, thus avoiding black spots.
  • Regular health inspections: Frequently checking your molly fish for any changes in behavior or appearance can assist in the early identification of problems such as black spots.
  • Quarantine fresh additions: Isolating new fish for 2-4 weeks prior to introducing them to your main tank can preclude the transfer of diseases like black spot disease to your existing molly fish.


For those of you who are short on time, here is a brief overview of what I discussed earlier:

  • Elevated ammonia levels in the aquarium stress molly fish, resulting in unsightly black spots on their bodies.
  • Black spot disease, caused by the Diplostomum parasite, spreads rapidly in contaminated water, leading to the formation of black cysts on molly fish.
  • Injuries, tank environment, aggressive tankmates, and improper handling can cause wounds that evolve into black spots during the healing process.
  • Genetic factors can contribute to the development of black spots in molly fish, with certain breeds or genetic variations triggering their formation.
  • To treat black spots, addressing ammonia poisoning, using antiparasitic remedies, promoting wound recovery, considering genetic factors, and maintaining optimal tank conditions are essential strategies.