Why Is My Molly Fish Gasping For Air? (With 5 Solutions)

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About two years ago I caught my molly fish gasping for air. At first, I thought it was merely looking for food. However, it didn’t take long for me to realize that something was wrong.

As it turned out, this behavior typically indicates that the fish is suffering. Fortunately, after extensive research, I was able to fix that by following some simple steps.

In this article, I will explain why mollies tend to gasp for air at the surface, show you how to stop this issue, and how to prevent it from happening again in the future.

Let’s get started.

Why Is My Molly Fish Gasping?

Molly fish typically gasp for air due to oxygen deficiencies in the water. The primary causes for this include:

1. Insufficient Filtration

Insufficient filtration can reduce oxygen levels in the aquarium, resulting in molly fish gasping due to oxygen deficiency.

Furthermore, inefficient filters may allow the accumulation of harmful substances that can impair oxygen exchange in molly fish.

Here are the key points to consider:

  • Oxygen Deprivation: Insufficient filtration can cause oxygen deprivation in molly fish, making them gasp for air at the surface due to a lack of oxygen in the water.
  • Harmful Substances: The accumulation of toxic substances like ammonia from inefficient filtration can impair the gills’ oxygen exchange process in molly fish.
  • Proper Filter Size: Choosing a filter proportional to your aquarium size is crucial to maintain sufficient oxygen levels for molly fish.
  • Filter Maintenance: Regular cleaning and maintenance of the filter can prevent blockages and ensure efficient oxygen circulation for molly fish.
  • Use of Air Stones: Air stones help in oxygenation of water, alleviating the oxygen deficiency in tanks with insufficient filtration.

2. Excessive Population

An overpopulated tank can deplete oxygen levels faster, leading to molly fish gasping due to a lack of available oxygen.

Molly fish sharing a tank with other species that are heavy oxygen consumers can worsen this issue.

Consider the following:

  • Increased Oxygen Demand: A high population of fish, including molly fish, can consume oxygen more rapidly, leading to oxygen deficiency.
  • Increased Waste: More fish means more waste, which consumes oxygen during breakdown, further reducing available oxygen for molly fish.
  • Species Compatibility: If molly fish share the tank with heavy oxygen-consuming species, it can aggravate the oxygen deficiency.
  • Population Control: To ensure sufficient oxygen, maintain a balance of one inch of adult fish per gallon of water, considering molly fish and other species.
  • Stress from Overcrowding: Overcrowding can stress molly fish, increasing their oxygen consumption and leading to gasping.

Also Read: Stress In Molly Fish

3. New Aquarium Syndrome

New tanks often lack the established beneficial bacteria that help break down fish waste, which can cause spikes in harmful substances, depleting oxygen and causing molly fish to gasp.

Here is what you should know:

  • Cycling Process: The new aquarium’s cycling process can lead to oxygen depletion, causing molly fish to gasp for air.
  • Harmful Substance Spikes: Spikes in ammonia and nitrite in new tanks can deplete oxygen levels, leading to oxygen deficiency in molly fish.
  • Beneficial Bacteria: Lack of beneficial bacteria in a new tank to break down waste can lead to oxygen depletion, stressing molly fish.
  • Use of Bacteria Cultures: Introducing commercial start-up bacteria cultures can help establish the nitrogen cycle faster, preventing oxygen depletion for molly fish.
  • Regular Testing: Regularly testing the water parameters in a new tank can help manage oxygen levels and prevent gasping in molly fish.

4. Damage to the Gills

When a molly fish’s gills are damaged, their capability to take in oxygen from water can be compromised, resulting in gasping.

This damage might be a consequence of hostility from fellow fish, improper handling, or infections.

Take note of the following:

  • Hindered Oxygen Uptake: Damage to the gills can hinder a molly fish’s capacity to obtain oxygen from water, causing them to gasp for air.
  • Hostility from Aquarium Companions: Gills can be harmed by hostile companions in the tank, causing oxygen shortage in molly fish.
  • Consequences of Poor Handling: Inappropriate handling during aquarium upkeep or repositioning can result in gill damage, leading to oxygen insufficiency in molly fish.
  • Infections by Bacteria and Parasites: Infections can inflame or harm the gills, affecting the molly fish’s ability to respire, which leads to gasping.
  • Healing Supports: Employing healing supports specifically created for aquarium fish can aid molly fish in recovering from gill damage, thus restoring their oxygen uptake capacity.

5. Methemoglobinemia

Elevated nitrite levels can cause Methemoglobinemia, or Brown Blood Disease, in molly fish. 

This condition hampers the blood’s ability to transport oxygen effectively, causing molly fish to gasp at the surface.

Here’s what you need to understand:

  • Nitrite-Triggered Oxygen Shortage: High nitrite levels can lead to Brown Blood Disease, preventing the molly fish’s blood from efficiently carrying oxygen, resulting in oxygen deprivation and gasping.
  • Routine Water Examination: Regularly examining your aquarium water for nitrite levels can assist you in recognizing and preventing Brown Blood Disease in molly fish.
  • Frequent Water Exchanges: Conducting regular water exchanges can help keep nitrite levels at a safe threshold, ensuring sufficient oxygen supply for molly fish.
  • Nitrite-Lowering Products: Using nitrite-lowering products can help preserve water quality and avoid oxygen deprivation in molly fish.
  • Observing Symptoms: Closely monitoring your molly fish for gasping signs can help detect Brown Blood Disease early, allowing for immediate intervention to avert severe oxygen shortage.

Also Read: Molly Fish Staying At The Top Of The Tank

Treating Air-Gasping in Molly Fish

Fortunately, there are practical steps you can take to stop your molly fish from gasping. Here is what you can do:

1. Addressing Insufficient Filtration

Air-gasping in molly fish can often be a result of insufficient filtration leading to a build-up of harmful compounds in the water.

A well-filtered, clean environment is vital for the health and well-being of your molly fish. Here is what you should know:

  • Proper Filter Selection: Choose a filter that can handle approximately 4-5 times the volume of your tank per hour. For instance, for a 100-gallon tank, opt for a filter with a flow rate of 400-500 gallons per hour. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Routine Filter Maintenance: Cleaning the filter every two weeks prevents clogging and keeps it functioning optimally.
  • Regular Water Changes: Conduct 20-25% water changes weekly to maintain the water quality and reduce the load on the filter.
  • Frequent Water Testing: Regularly testing water parameters (every week) ensures levels of ammonia, nitrites, and nitrates are in check. Aim for 0 ppm for both ammonia and nitrites and less than 20 ppm for nitrates. I personally use the API FRESHWATER MASTER TEST (link to Amazon).
  • Monitoring Fish Behavior: Watch out for abnormal behaviors like gasping for air, indicating poor water conditions possibly due to insufficient filtration.

Also Read: Do Molly Fish Need A Filter?

2. Managing Excessive Population

Overcrowding can lead to stressful conditions causing molly fish to gasp for air. Adequate tank space per molly fish ensures less stress and better health.

Consider these points:

  • Ideal Fish-to-Space Ratio: A general rule is to allow 1 gallon of water for every inch of a fully-grown molly fish. Overcrowding leads to stress, so maintaining this ratio is critical.
  • Quarantine New Additions: Quarantine new fish for at least 2 weeks before introducing them to the tank to prevent the spread of diseases.
  • Birth Control: Mollies breed rapidly. A single female can give birth to 20-100 fry every 60-70 days. To prevent overpopulation, consider separating males and females or use birth control methods if available.
  • Rehoming or Selling Fish: If your molly population exceeds the capacity of your tank, consider giving some fish to other hobbyists or selling them to a local pet store.
  • Fish Mortality Rate: In a well-maintained tank, mollies have a survival rate of about 80%. Overpopulation could decrease this percentage, indicating the need for better population management.

Also Read: Molly Fish Tank Size

3. Dealing with New Aquarium Syndrome

New aquarium syndrome can lead to an ammonia spike, causing molly fish to gasp for air due to lack of oxygen. Proper tank cycling is essential before introducing molly fish.

Here is what you should do:

  • Proper Tank Cycling: A new tank should be cycled for at least 4-6 weeks before introducing molly fish to establish beneficial bacteria that convert harmful waste.
  • Use of Water Conditioners: Add water conditioners to neutralize harmful chlorine and chloramine in tap water. My recommendation: Tetra AquaSafe (link to Amazon).
  • Monitoring Water Parameters: Regularly test the water, especially for ammonia, nitrites, and nitrates. The goal is to achieve 0 ppm for ammonia and nitrites, and less than 20 ppm for nitrates.
  • Slow Introduction of Fish: Introduce molly fish slowly, ideally only 2-3 at a time, to not overwhelm the new tank’s bacterial balance.
  • Use of Starter Bacteria Cultures: These can help speed up the cycling process by adding beneficial bacteria to the tank directly.

Also Read: Molly Fish Tank Setup

4. Remedying Gill Injuries

Gill injuries can cause molly fish to gasp for air. Causes include sharp objects in the tank and aggressive tank mates.

Consider these precautions:

  • Careful Decoration Selection: Choose smooth, non-abrasive decorations to prevent accidental injuries. Avoid anything with sharp edges.
  • Regular Observation: Monitor your molly fish daily for any signs of injury or distress.
  • Immediate Isolation: If a molly fish shows signs of gill injury, isolate it immediately to prevent further harm and to allow for healing.
  • Proper Treatment: Consult a fish health professional or use over-the-counter fish medication to treat gill injuries. Always follow recommended dosages to avoid overdosing. My recommendation: API Stress Coat (link to Amazon).
  • Peaceful Tank Mates: Mollies are peaceful fish. Avoid housing them with aggressive species that might injure them. Mollies should constitute at least 70% of the tank population to ensure a peaceful environment.

For example, here are some peaceful species that can happily live with mollies:

  • Platyfish (Xiphophorus maculatus)
  • Corydoras Catfish (Corydoras spp.)
  • Guppy (Poecilia reticulata)
  • Swordtail (Xiphophorus hellerii)
  • Endler’s Livebearer (Poecilia wingei)
  • Cherry Barb (Puntius titteya)
  • White Cloud Mountain Minnow (Tanichthys albonubes)

On the other hand, avoid aggressive species like:

  • Betta Fish (Betta splendens)
  • African Cichlid (various species from the Cichlidae family)
  • Red-Tailed Shark (Epalzeorhynchos bicolor)
  • Jack Dempsey Cichlid (Rocio octofasciata)
  • Convict Cichlid (Amatitlania nigrofasciata)
  • Tiger Barb (Puntigrus tetrazona)
  • Siamese Algae Eater (Crossocheilus oblongus)

Also Read: 19 Great Neon Tetra Tank Mates

5. Treating Brown Blood Disease

Brown Blood Disease, caused by high nitrate levels, can make molly fish gasp for air. Regular monitoring and maintenance can prevent this condition.

Here is what you should know:

  • Regular Water Changes: Conducting a 20-25% water change every week can help keep nitrate levels below 20 ppm.
  • Testing Water Quality: Regularly test the water (at least once a week) to keep track of nitrate levels.
  • Plant Addition: Live plants can help absorb excess nitrates, providing a natural solution to nitrate accumulation.
  • Less Overfeeding: Overfeeding can result in extra waste, leading to higher nitrate levels. Feed only an amount your mollies can consume in 2-3 minutes.
  • Nitrate-Reducing Products: Use nitrate-reducing products or filters as per the instructions if nitrates persistently stay high.

Also Read: 15 Molly Fish Diseases & Their Treatments

Preventing Molly Fish from Gasping in the Future

Preventing molly fish from gasping in the future primarily involves maintaining a clean, stress-free environment with the right water parameters.

Consistent care, routine checks, and preemptive measures are key to achieving this goal. Here’s what you should consider:

  • Consistent Water Testing: Regular testing, preferably once a week, helps you maintain optimal water conditions for your molly fish (0 ppm for ammonia and nitrites, and less than 20 ppm for nitrates).
  • Adequate Filtration: A filtration system that can handle 4-5 times the volume of your tank per hour is recommended.
  • Balanced Diet: Feeding your molly fish a varied diet of flakes, live food, and vegetables, but only as much as they can consume in 2-3 minutes, ensures their optimal health.
  • Population Management: Keep the population in check. For every inch of a fully-grown molly fish, consider providing 1 gallon of water to avoid stress due to overcrowding.
  • Regular Observation: Monitor your molly fish daily. Behavioral changes such as air-gasping can often indicate a problem in its early stages, allowing for prompt action.


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

  • Molly fish gasp for air primarily due to oxygen deficiencies in the water, which can be caused by insufficient filtration and the accumulation of harmful substances.
  • Overpopulation in the tank, including molly fish and other oxygen-consuming species, can deplete oxygen levels faster, leading to gasping.
  • New aquarium syndrome, characterized by the lack of established beneficial bacteria, can cause spikes in harmful substances and oxygen depletion in molly fish.
  • Damage to the gills from aggression, improper handling, or infections can impair oxygen uptake, resulting in gasping behavior.
  • High nitrite levels can lead to Methemoglobinemia, affecting the blood’s ability to carry oxygen and causing molly fish to gasp. Regular water testing and maintenance are crucial to prevent this condition.