How Many Molly Fish In A 40-Gallon Tank? (& Other Fish)

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Mollies are a great choice if you are looking for new fish for your 40-gallon tank. They are both beautiful and peaceful, and you aren’t likely to face any issues growing them.

But how many molly fish can you actually keep in 40 gallons? How many is too much? Will breeding cause any issues? And what about other fish species to keep with them?

In this article, I will discuss all these questions and many others. So, without further ado, let’s get started.

How Many Molly Fish Can You Keep In 40 Gallons?

In a 40-gallon tank, you can comfortably house about eight to ten adult molly fish, considering their social nature and requirement for sufficient space to swim and explore. 

This follows the 1-inch per gallon rule, taking into account their mature size.

However, it’s important to monitor the tank conditions and fish behaviors to ensure it doesn’t become overpopulated.

How Do I Calculate The Appropriate Quantity Of Molly Fish For A 40-Gallon Tank?

Here is how to calculate the number of mollies you can keep in a tank of 40 gallons, following the one-inch-per-gallon rule:

  • Identify the mature size of the molly fish you intend to keep; mollies typically reach a size of 4 to 5 inches when fully grown.
  • Understand the 1-inch-per-gallon rule, which states you should allocate one gallon of water per inch of fully grown fish.
  • Apply this rule by multiplying the gallon capacity of your tank. For a 40-gallon tank, this equates to 40 inches of fish.
  • Divide this total by the mature size of the molly fish. If we use the maximum size of 5 inches, we get 40 divided by 5, which equals 8 molly fish.
  • Round down if necessary, ensuring there’s enough space for all the fish. So in a 40-gallon tank, you should be able to house 8 to 10 adult molly fish comfortably.

Why Is A 40-Gallon Tank Considered Ideal For Molly Fish?

A 40-gallon tank is considered ideal for molly fish because it provides ample space for these active swimmers to thrive and also facilitates a stable environment, which mollies prefer. 

This tank size suits their unique requirements and natural behaviors, creating a more comfortable and healthier environment for them.

Here are some supporting points:

  • Spacious Environment: Molly fish love to explore. A 40-gallon tank offers plenty of room for their active swimming and exploring habits.
  • Social Dynamics: Mollies thrive in groups. A 40-gallon tank can comfortably house 8 to 10 mollies, allowing for healthy social interaction.
  • Stable Water Conditions: Larger tanks like 40 gallons help maintain stable water conditions. Stable temperatures and pH levels are crucial for molly fish health.
  • Growth and Reproduction: Molly fish breed frequently. A 40-gallon tank can accommodate this breeding while providing space for the fish to grow.
  • Decoration and Landscaping: Mollies enjoy tanks with decorations. A 40-gallon tank allows ample space for adding plants, rocks, and other items without cramping their space.

Also Read: Molly Fish Tank Size

Maintenance Tips For A 40-Gallon Tank With Molly Fish

Maintaining a 40-gallon tank with molly fish involves regular water changes, monitoring water parameters, and providing a balanced diet among other steps.

Proper maintenance ensures the health and longevity of your mollies. Let’s delve into some critical maintenance tips:

  • Regular Water Changes: Regularly replace 25-30% of the water every week. This ensures optimal water quality for molly fish.
  • Monitor Water Parameters: Check water temperature, pH, and hardness frequently. Mollies prefer a temperature between 72-78°F, and slightly alkaline water (pH 7.5-8.5).
  • Balanced Diet: Feed mollies a varied diet of flakes, pellets, and occasional live food. This supports their health and colorful appearance.
  • Tank Cleaning: Clean the tank decorations and walls without disturbing the fish. This keeps the tank healthy and enjoyable for molly fish.
  • Check Health Regularly: Look out for signs of illness such as lethargy, loss of appetite, or color fading. Early detection is key to successful treatment.
  • Tank Mates: Choose tank mates wisely. Not all fish species are compatible with mollies, so research before adding new fish.

Also Read: How Many Molly Fish In A 30-Gallon Tank?

What If You Add Too Many Molly Fish To A 40-Gallon Tank?

Exceeding the recommended number of molly fish in a 40-gallon tank can result in overcrowding, leading to fish stress and potential health problems.

It’s essential to adhere to the appropriate stocking levels to maintain a healthy tank environment.

Let’s look at the potential problems of overstocking:

  • Stress Inducing: Overcrowding can cause stress in molly fish, leading to decreased immunity and increased susceptibility to diseases.
  • Poor Water Quality: Too many fish increase waste production, leading to high nitrate levels and poor water conditions, which are harmful to molly fish.
  • Limited Resources: Overcrowding can lead to competition for food, resulting in malnutrition for some mollies.
  • Aggression: Limited space can trigger territorial behavior, causing mollies to become aggressive, leading to physical harm.
  • Breeding Issues: Overcrowded conditions may hinder proper breeding behavior in mollies, affecting their reproduction.

Does The Gender Of Molly Fish Matter In A 40-Gallon Setup?

The gender of molly fish doesn’t significantly affect the setup of a 40-gallon tank as both male and female mollies have similar needs and behaviors.

To further elaborate:

  • Behavior: Both male and female mollies have social behavior. They all enjoy interacting with others and prefer being in groups in the 40-gallon setup.
  • Size: Male and female mollies generally grow to a similar size, so they require equivalent amounts of space in a 40-gallon tank.
  • Environmental Needs: Both genders need the same water conditions, diet, and decor in the tank. Hence, the gender doesn’t affect the setup of a 40-gallon tank.

Can Molly Fish Be Bred In A 40-Gallon Tank?

Yes, a 40-gallon tank is indeed adequate for breeding molly fish, as long as the environment and conditions are conducive.

Such a tank size offers plenty of room for the fish to breed and for their offspring to grow. To facilitate successful breeding of molly fish in a 40-gallon tank:

  • Secure Hideaways: Incorporate live plants or other forms of cover for the fry to shelter and avoid adult mollies.
  • Gender Ratio: Maintain a female-to-male ratio that leans heavier on females, typically 2 or 3 females per male, to lessen the stress on the female mollies.
  • Nutrition: Administer a diet rich in nutrients and quality, ensuring the breeding mollies and their fry are adequately nourished.
  • Ideal Water Conditions: Ensure the water parameters, such as temperature, pH, and hardness, are maintained within the optimal range for mollies.
  • Segregation: If necessary, think about employing a breeding tank or box to safeguard the fry post-birth.

Also Read: How Many Molly Fish In A 55-Gallon Tank?

Is It Possible To Add More Molly Fish To A 40-Gallon Tank?

Introducing additional molly fish to a 40-gallon tank is feasible, but it’s critical to do it carefully to avoid overstocking and reduce fish stress.

In order to introduce more mollies into your tank:

  • Adhere to the Rule: Bear in mind the 1-inch-per-gallon rule and make sure your tank isn’t at full capacity already.
  • Isolation of New Fish: Prior to introduction, isolate the new fish for a period of 2 weeks to preclude any disease transmission.
  • Gradual Introduction: Add new mollies gradually over time to prevent abrupt shifts in water conditions.
  • Monitoring of Conditions: Keep an eye on water parameters frequently, as an increased number of fish can heighten waste levels and affect water quality.
  • Compatibility: Ensure that the new mollies will coexist well with the existing fish to prevent any aggressive or stressful scenarios.

How Many Molly Fish Should Be Kept Together In General?

Molly fish thrive best in a social setting, and generally, a group of 4-5 molly fish is recommended for a balanced community.

This number supports their social interactions and natural behaviors such as schooling and mating, while minimizing the risk of overcrowding and intense competition for resources. 

Always ensure the tank size is appropriate for the number of fish, and remember to account for their full-grown size.

What Other Fish Can Be Kept With Molly Fish In A 40-Gallon Tank?

After figuring out how many mollies you can keep in 40 gallons, you probably wonder what other fish get along with them, and how many of those you should get.

Here is a simple table discussing some of the most common species:

Fish SpeciesNumber of Fish (40 Gallons)
Molly Fish Only8-10 mollies
Molly Fish and Guppies8 mollies, 15-20 guppies
Molly Fish and Neon Tetras8 mollies, 20-25 neon tetras
Molly Fish and Zebra Danios8 mollies, 15-20 zebra danios
Molly Fish and Platy Fish8 mollies, 10-15 platy fish
Molly Fish and Betta Fish8 mollies, 1 betta fish

1. Molly Fish And Guppies

In a 40-gallon tank, you can comfortably house around 8 mollies along with 15-20 guppies. 

Both species share similar water requirements and are generally peaceful, making them a good combination.

2. Molly Fish And Neon Tetras

You can keep around 8 mollies with a school of 20-25 neon tetras in a 40-gallon tank. Neon tetras are small and peaceful, making them suitable companions for mollies.

3. Molly Fish And Zebra Danios

In a 40-gallon setup, about 8 mollies and 15-20 zebra danios can cohabitate. Zebra danios are active swimmers like mollies, and their peaceful nature makes them good tank mates.

4. Molly Fish And Platy Fish

A mix of approximately 8 mollies and 10-15 platy fish can be kept together in a 40-gallon tank.

Both species are livebearers with similar needs and behaviors, making them compatible.

5. Molly Fish And Betta Fish

While bettas are often territorial, a 40-gallon tank provides ample space for 1 betta fish along with 8 mollies.

Always monitor behavior to ensure the betta doesn’t become aggressive towards the mollies.


Let’s quickly summarize what I discussed earlier:

  • Keeping 8 to 10 adult molly fish is recommended for a 40-gallon tank to allow for social interaction and prevent overcrowding and stress.
  • The 1-inch per gallon rule is used to calculate the appropriate number of mollies for a 40-gallon tank, considering their mature size.
  • Maintaining a 40-gallon tank is ideal for molly fish as it provides ample space for their active swimming, social dynamics, and growth.
  • Regular maintenance of a 40-gallon tank with molly fish includes water changes, monitoring water parameters, balanced diet, and regular health checks.
  • Adding too many molly fish to a 40-gallon tank can lead to overcrowding, stress, poor water quality, limited resources, and breeding issues.