Molly Fish Size: How Big Do They Get?

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Many fish owners want to know how big their fish will get and for a good reason – each fish grows to a different size and occupies a different amount of space.

In this article, I will focus on molly fish and discuss everything there is to know about their average size.

You’ll learn how big mollies can get, what the average size of different mollies species is, what can stunt their growth, and how to ensure they grow properly.

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

The Average Molly Fish Size

The average size of a molly fish largely depends on the species, typically ranging between 2 to 4 inches in length.

Some specific species, such as the sailfin molly, can grow as large as 5 inches in optimum conditions.

Here is a brief explanation of what you should know:

  • Species Impact: Different species of molly fish show varied sizes. For example, the short-finned molly fish generally measures around 3 inches, while the sailfin molly fish, in optimal conditions, can attain a length of 5 inches.
  • Gender Differences: Male molly fish are often slightly smaller in comparison to females. This difference, though not very significant, is quite a common trait found across multiple molly fish species.
  • Environmental Factors: The size of a molly fish is influenced by the environment, including tank conditions and diet. Access to a nutritious diet and spacious living conditions can allow a molly fish to reach its maximum possible size.

Here’s a concise table that illustrates the average length range for each of the most common molly species:

Molly SpeciesAverage Length Range (inches)
Sailfin Molly4-5
Mexican Sailfin Molly4
Yucatan Molly3-4
Shortfin Molly2-3
Black Molly2-3
Liberty Molly2-3
Balloon Molly1-2
Golden Sailfin Molly1-2
Dalmatian Molly1-2
Marbled Molly1-2

Also Read: 15 Things You Should Know About Molly Fish

Molly Fish Size By Age

The size of a molly fish grows progressively with its age, starting from being a tiny fry at birth to reaching full size as an adult.

During their first few weeks, molly fish are relatively small and experience rapid growth. The rate of growth slows down as they approach adulthood around the 6 to 8-month mark.

Here’s a table that depicts the average size of a molly fish at various ages:

AgeAverage Size (inches)
1 week0.2
3 weeks0.5
1 month0.75
3 months1.5-2
6 months2-2.5
8 months2.5-3

Is There A Size Difference Between Male And Female Molly Fish?

Yes, there is generally a size difference between male and female molly fish, with females often being larger. This difference is subtle but consistent across most molly fish species.

To further elaborate on this point:

  • Size Disparity: Female molly fish usually grow slightly larger than males, often reaching an average length of 3-4 inches, while males typically measure around 2-3 inches.
  • Reason for Size Difference: This size difference can be attributed to the biological need for females to carry and birth offspring.
  • Appearance: Beyond size, males and females also exhibit different physical characteristics. Males, while smaller, are often more colorful and have longer fins.
  • Species Variation: While this size difference is a general trend, certain species of molly fish may show less pronounced size disparities.
  • Age Factor: It’s important to note that young molly fish, irrespective of gender, are roughly the same size until they reach maturity.

Also Read: Male vs. Female Molly Fish

When Does a Molly Fish Stop Growing?

Molly fish usually stop getting bigger when they hit maturity, which typically happens when they’re between 6 and 8 months old.

That said, the exact timing can be influenced by things like what they eat, where they live, and the traits they’ve inherited.

Once they reach their full size, the pace at which molly fish grow slows down a lot, and any extra growth is usually just a tiny bit.

Why Some Molly Fish Stop Growing

Even though mollies can potentially reach a size of 2 to 4 inches, not all of them will:

1. Insufficient Tank Size & Environment

Molly fish need an adequate tank size and environment for optimal growth. A small, cramped tank can limit their size and overall health:

  • Tank Size: Molly fish, particularly larger species, require tanks of at least 20-30 gallons to grow to their full size.
  • Environment Impact: Without ample space and environmental enrichment, molly fish cannot exercise and grow properly.
  • Other Inhabitants: Overcrowding in the tank can also cause stunted growth, as the fish may compete for resources.

2. Poor Water Quality

Good water quality is crucial for the growth and overall health of molly fish. Poor water conditions can lead to stress and diseases, which inhibit growth.

Here’s how water quality impacts molly fish:

  • Chemical Balance: Maintaining appropriate pH, ammonia, nitrite, and nitrate levels is key for molly fish growth.
  • Temperature: The water temperature should be kept between 72°F and 82°F for molly fish to thrive.
  • Regular Cleaning: Keeping the tank clean prevents the buildup of harmful substances that can stunt growth.

3. Inadequate Diet

Molly fish require a varied, balanced diet for proper growth. An inadequate diet can result in malnutrition and stunted growth:

  • Diet Variety: Molly fish are omnivores and their diet should include both plant and animal matter.
  • Nutrient Rich Foods: High-quality commercial foods, live foods, and vegetables provide essential nutrients for growth.
  • Feeding Schedule: Overfeeding or underfeeding can both adversely affect a molly fish’s growth.

4. Stress

Stress is a significant factor that can stunt the growth of molly fish. It can arise from various sources and negatively affect their immune system.

Here’s how stress influences growth:

  • Environmental Stressors: Changes in water temperature, poor water quality, or an overcrowded tank can induce stress.
  • Social Stress: Aggression from tankmates can cause stress, leading to stunted growth.
  • Handling Stress: Frequent handling or moving can stress molly fish, inhibiting their growth.

5. Disease

Disease can significantly impact the growth of molly fish. It often results from stress, poor water conditions, or an inadequate diet.

Here’s how disease affects growth:

  • Common Diseases: Diseases like Ich or fin rot can stunt growth as the molly fish’s energy is used to fight off the infection.
  • Impact of Disease: Sick molly fish often have reduced appetites, leading to malnutrition and stunted growth.
  • Prevention: Regular tank maintenance, a balanced diet, and a stress-free environment help prevent diseases.

6. Genetic Factors

Some molly fish may have genetic traits that limit their growth. This factor is beyond the control of the fish keeper. Here are a few things to keep in mind:

  • Inherited Traits: Some mollies may inherit traits from their parents that limit their maximum size.
  • Breed Variation: Certain breeds of molly fish are naturally smaller, regardless of their environment or diet.
  • Unpredictability: Genetics can be unpredictable and can sometimes lead to unexpected growth patterns.

7. Age & Growth Limitations

Molly fish generally stop growing upon reaching maturity. Each fish has a biological growth limit, typically reached at around 6-8 months:

  • Growth Stages: Molly fish grow rapidly as juveniles but this growth slows as they approach adulthood.
  • Maturity: Once they reach maturity, molly fish growth slows significantly and they reach their full size.
  • Lifespan: The average lifespan of a molly fish is around 3-5 years, during which the size remains relatively constant after maturity.

How To Make Sure Your Molly Fish Grow Properly

Taking care of your mollies is crucial if you want them to grow properly. Here are some steps to consider:

1. Provide a Suitable Tank Size

Molly fish require adequate space to grow and thrive. An appropriate-sized tank is necessary to provide enough room for these fish to swim freely:

  • Tank Size: A 20-30 gallon tank is recommended for molly fish as the base size. Each molly needs about 2-3 gallons, providing sufficient space for swimming.
  • Tankmates: Maintain a 3:1 female to male molly fish ratio to prevent the males from harassing the females, thus reducing stress and aggression.
  • Quantity: With the ‘one inch per gallon’ rule, a standard 20-gallon tank could ideally accommodate about 6-10 adult mollies, depending on their size.
  • Decor: Mollies appreciate dense vegetation and hiding spots, so introduce plants, rocks, and caves to replicate their natural habitat.
  • Room for Growth: As mollies can grow up to 4.5 inches, ensure the tank has enough room to accommodate their full-grown size.

Also Read: Molly Fish Tank Size

2. Adjust the Environment

Creating the right environmental conditions is crucial for the molly fish’s well-being and growth. An incorrect environment can stunt growth or cause illness.

Here is what you should do:

  • Temperature: Maintain a temperature range of 72-82°F (22-28°C), which is ideal for molly fish’s metabolism and growth.
  • Lighting: Mollies require around 8-12 hours of light per day to mimic a natural day-night cycle.
  • pH Level: Molly fish thrive in a pH range of 7.5-8.5, replicating their natural brackish water environment.
  • Salinity: Although mollies can survive in freshwater, adding aquarium salt (about 1-1.5 teaspoons per 5 gallons) can enhance their health and growth.
  • Substrate: A sandy substrate mimics their natural environment and prevents injury while they forage.

Also Read: Molly Fish Tank Setup

3. Maintain Proper Water Quality

Molly fish are sensitive to water quality, and poor conditions can hinder their growth. Regular water changes and testing are essential:

  • Water Changes: Conduct a 25-50% water change weekly to reduce ammonia, nitrite, and nitrate levels, thus promoting molly fish health.
  • Filtration: A good filtration system that can handle biological, mechanical, and chemical filtration will ensure a clean environment for mollies. My recommendation: Fluval C4 Power Filter (link to Amazon).
  • Testing: Regular water testing (weekly or bi-weekly) for ammonia, nitrite, nitrate, pH, and salinity ensures optimal water conditions. I do that with the well-known API FRESHWATER MASTER TEST (link to Amazon).
  • Avoid Overfeeding: Overfeeding can pollute water quickly, so feed molly fish only what they can consume in 2-3 minutes.
  • Use Dechlorinator: Always treat tap water with a dechlorinator before adding to the tank, as chlorine is harmful to molly fish. My recommendation: Tetra AquaSafe (link to Amazon).

4. Offer a Balanced and Nutritious Diet

Molly fish need a varied diet for proper growth. Supplying them with a mixture of commercial and live food will ensure balanced nutrition:

  • Varied Diet: Feed molly fish a mixture of high-quality flake food, brine shrimp, bloodworms, and vegetables to ensure a balanced diet.
  • Feeding Frequency: Feeding them small portions 2-3 times a day will prevent overeating and water pollution.
  • Live Foods: Introduce live foods like daphnia and tubifex worms once a week to mimic their natural diet, improving growth.
  • Vegetables: Supplement their diet with blanched vegetables like peas, lettuce, or zucchini twice a week for additional fiber.
  • Vitamins & Minerals: Consider food fortified with vitamins and minerals to boost molly fish immunity and promote healthy growth.

Also Read: How To Feed Molly Fish

5. Keep Stress Levels Low

Stress can seriously inhibit molly fish growth. To ensure they’re living stress-free, a peaceful environment is crucial:

  • Peaceful Surroundings: Situate the tank in a tranquil spot to prevent surprising your mollies and promote their healthy growth.
  • Steer Clear of Overcrowding: Too many fish can lead to stress and disease, so stick to an optimal number of mollies in the tank.
  • Gentle Handling: When you need to handle them, opt for a soft mesh net to avoid hurting their scales and causing stress.
  • Incremental Adjustments: Implement changes to the tank conditions (like temperature, pH, etc.) in a gradual manner to avoid stress and shock to your mollies.
  • Tank Mates: Choose harmonious tank mates like platies, guppies, and tetras to minimize conflict and keep stress levels low for molly fish.

Also Read: 19 Great Molly Fish Tank Mates

6. Disease Prevention and Treatment

Molly fish can be prone to a number of diseases, which can inhibit their growth. Regular checks and timely intervention can ward off health problems.

Take note of these points:

  • Isolate New Arrivals: Quarantine any new fish for 2-4 weeks prior to adding them to the main tank to prevent disease spread.
  • Identify Common Illnesses: Be vigilant for symptoms of common molly diseases like Ich, Fin Rot, and Velvet, and provide immediate treatment.
  • Essential Medications: Maintain a basic aquarium medicine kit, which includes remedies for bacterial and parasitic ailments common to mollies.
  • Routine Check-ups: Observe your mollies on a daily basis for signs of discomfort or unusual behavior, these might be signs of health issues.
  • Seek Professional Advice: If a disease appears severe or doesn’t respond to simple treatments, don’t hesitate to consult a vet.

Also Read: 15 Molly Fish Diseases & Their Treatments

7. Select Healthy Specimens

Choosing healthy molly fish is crucial to ensuring they grow properly. Observing their behavior and appearance can help identify healthy individuals:

  • Active Behavior: Choose molly fish that are active and alert, swimming around the tank with a smooth motion.
  • Physical Appearance: Healthy molly fish have bright, clear eyes, intact scales, and full, vibrant fins.
  • Avoid Thin Fish: Avoid mollies that look thin or have a sunken belly as these could indicate health problems or malnourishment.
  • Check for Diseases: Look for signs of common diseases like Ich (white spots), Velvet (gold dust), or fin damage before purchase.
  • Good Breeder/Store: Buy from a reputable breeder or pet store that maintains proper fish health and tank hygiene to ensure the health of your molly fish.


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

  • Molly fish come in various species, with an average size ranging from 2 to 4 inches, but certain types, like the sailfin molly, can reach up to 5 inches in optimal conditions.
  • Male molly fish tend to be slightly smaller than females, a common trend observed across different molly species.
  • Factors such as environment, diet, and genetics play a significant role in determining the size of a molly fish, which typically stops growing around 6 to 8 months of age.
  • Proper care, including providing a suitable tank size, balanced diet, and minimizing stress, is crucial for ensuring healthy growth and development in molly fish.
  • Inadequate tank size, poor water quality, insufficient diet, stress, disease, and genetic traits are common factors that can lead to stunted growth in molly fish.