How Many Molly Fish In A 20-Gallon Tank? (With Examples)

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If you have just received your 20-gallon tank and wonder how many mollies you can raise in it, you have come to the right place.

In this article, I will discuss how many molly fish you can accommodate in that tank size, and provide some useful tips on how to grow them properly in 20 gallons.

You will also learn what types of fish you can mix with mollies in that tank size, and whether breeding is actually feasible.

Let’s get started.

How Many Molly Fish Can Be Kept Together In 20 Gallons?

For a 20-gallon tank, you can safely accommodate four to five adult molly fish.

Molly fish are social creatures and prefer to be in groups, but they also need enough space to swim freely, which is why the 1-inch per gallon rule generally applies, taking into consideration their full-grown size.

Calculating The Appropriate Number Of Molly Fish For A 20-Gallon Tank

Here is how you can calculate how many mollies can live in 20 gallons, following the one-inc-per-gallon rule:

  • The 1-inch-per-gallon rule recommends one inch of fish per gallon of water.
  • Molly fish can grow up to approximately 4.5 inches.
  • Divide the tank capacity (20 gallons) by the adult size of molly (4.5 inches). The calculation is 20 gallons divided by 4.5 inches, resulting in roughly 4.4.
  • Round down to the nearest whole number. Therefore, a 20-gallon tank could comfortably house 4 full-grown molly fish based on the 1-inch-per-gallon rule.

Why Is A 20-Gallon Tank Considered The Minimum Size For Molly Fish?

A 20-gallon tank is considered the minimum size for molly fish because it provides the necessary space for their natural behavior and physical growth.

This optimal tank size contributes to the overall well-being and longevity of the molly fish.

Let’s delve into the reasons:

  • Swimming Space: Molly fish are active swimmers, requiring a spacious environment to maneuver comfortably. A 20-gallon tank provides ample room for such activities.
  • Population Control: Molly fish are prolific breeders; having a larger tank like a 20-gallon one helps to manage the population, preventing overcrowding and stress amongst fish.
  • Growth Accommodation: Adult molly fish can grow up to 4.5 inches. A 20-gallon tank gives them sufficient space to grow and develop without restrictions.
  • Water Quality: Larger volumes of water, like in a 20-gallon tank, can dilute waste and toxins more effectively, maintaining a healthier environment for the molly fish.
  • Social Interaction: Molly fish are social creatures. A 20-gallon tank provides the necessary space for group interaction, important for their well-being.

Also Read: Molly Fish Tank Size

Tips For Successfully Raising Molly Fish In A 20-Gallon Tank

Successfully raising molly fish in a 20-gallon tank involves maintaining proper water conditions, providing a balanced diet, and facilitating a comfortable environment for growth and interaction.

Adhering to specific guidelines can ensure the optimal health and longevity of molly fish.

Here is what you can do:

  • Water Parameters: Maintain a water temperature of 72-78°F, pH of 7.5-8.5, and a general hardness of 20-30 dGH for molly fish.
  • Balanced Diet: Feed molly fish a mix of high-quality flakes, pellets, and occasional live or frozen foods like brine shrimp.
  • Regular Cleaning: Perform 25-30% water changes weekly to maintain the water quality and provide a healthy environment for molly fish.
  • Tank Decor: Include a variety of plants and hideaways to replicate molly fish’s natural environment and promote comfortable living.
  • Lighting: Provide 12-14 hours of light daily, imitating the natural light cycle for molly fish.
  • Careful Introduction: Introduce new molly fish gradually to minimize stress and reduce chances of territorial conflicts.
  • Adequate Filtration: Use a high-quality filter to keep water clean and well-oxygenated for molly fish. My recommendation: Fluval C4 Power Filter (link to Amazon).

What If You Overstock Your Tank With Molly Fish?

Overstocking your tank with molly fish can lead to numerous problems including poor water quality, increased disease risk, and heightened aggression.

The stress of overcrowding can significantly affect the wellbeing of molly fish. Here’s a closer look at the potential issues:

  • Poor Water Quality: With more molly fish, waste production increases, leading to higher ammonia and nitrite levels that can poison fish.
  • Increased Disease Risk: Overcrowded conditions are stressful for molly fish, weakening their immune systems and making them more susceptible to disease.
  • Food Competition: More molly fish mean more competition for food, which can lead to malnutrition for some fish.
  • Stressed Fish: Limited space and resources can cause stress, impacting molly fish behavior and health negatively.
  • Aggression and Territory Issues: Overcrowding can provoke aggressive behavior among molly fish as they vie for territory.
  • Breeding Complications: Too many molly fish can lead to unchecked breeding, further exacerbating overcrowding issues.

Also Read: How Many Molly Fish In A 10-Gallon Tank?

Does The Gender Of Molly Fish Make A Difference?

Yes, the gender of molly fish does make a difference when determining how many mollies can be kept in a 20-gallon tank.

With their frequent breeding, a mix of male and female molly fish may lead to overcrowding quicker than a single-gender setup.

Hence, managing the gender ratio, usually favoring more females to males, is essential to control population growth in a 20-gallon tank.

Can You Breed Molly Fish In A 20-Gallon Setup?

Yes, you can breed molly fish in a 20-gallon setup as it provides adequate space for breeding and offspring growth.

However, successful breeding requires specific conditions and careful management. Here’s what you need to know:

  • Gender Balance: Maintaining a higher female-to-male ratio, such as 2:1, promotes better breeding behavior among molly fish and minimizes stress on female individuals.
  • Breeding Environment: Molly fish need dense plant coverage or breeding boxes to provide safe areas for females to release their fry.
  • Fry Care: Separate the fry from adult molly fish to prevent them from being eaten. A partition or separate tank can be useful.
  • Diet Consideration: Feed fry with nutrient-rich food like powdered flakes or baby brine shrimp to support their growth.
  • Water Conditions: Maintain optimal water parameters (temperature of 72-78°F, pH of 7.5-8.5) to ensure a healthy breeding environment for molly fish.

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

Adding More Molly Fish To A 20-Gallon Tank: Is It Possible?

Yes, you can add more molly fish to a 20-gallon tank by carefully managing the tank conditions and monitoring the fish behavior. Here’s how:

  • Gradual Addition: Add new molly fish one at a time, with a gap of at least a week between each addition. This gives the existing fish time to adjust and the tank’s bio-load to stabilize.
  • Quarantine New Fish: Quarantine new molly fish for about two weeks before introducing them to the main tank. This helps prevent the spread of potential illnesses within the community.
  • Monitor Aggression: Check for signs of aggression daily, like fin-nipping or chasing. If you notice these signs, it may be an indication that the tank is getting overcrowded or the male-to-female ratio is off.
  • Enhance Filtration: If adding more than 3-4 fish, consider upgrading to a filter rated for larger tanks to efficiently handle the increased waste.
  • Regular Water Changes: Increase your water changes to twice a week, replacing about 20-25% of the water each time, to help maintain water quality.
  • Optimize Space: Make sure each molly fish has at least a 2-inch radius of personal space. Use decorations and plants strategically to maximize available space and provide hiding spots.

Optimal Number Of Molly Fish To Keep Together

A group of 4-5 molly fish is often considered optimal to encourage social interaction and maintain a balanced community.

This number allows the fish to exhibit their natural behaviors, like schooling and mating, while preventing overcrowding and excessive competition for resources.

Keeping Molly Fish With Other Fish In A 20-Gallon Tank

Naturally, you would like to keep other fish species along with your mollies in your 20-gallon tank. Here are some common options:

Fish CombinationNumber of Fish (in 20 Gallons)
Only Molly Fish5-6 Molly Fish
Molly Fish + Guppies4-5 Molly Fish, 6-7 Guppies
Molly Fish + Neon Tetras4-5 Molly Fish, 10-12 Neon Tetras
Molly Fish + Zebra Danios4 Molly Fish, 6-8 Zebra Danios
Molly Fish + Platy Fish4 Molly Fish, 4-5 Platy Fish
Molly Fish + Betta Fish3-4 Molly Fish, 1 Betta Fish

1. Molly Fish And Guppies

In a 20-gallon tank, you can comfortably keep around 4-5 molly fish and 6-7 guppies. 

Guppies are smaller and have a peaceful temperament, making them good tankmates for molly fish.

2. Molly Fish And Neon Tetras

You could have 4-5 molly fish along with a school of 10-12 neon tetras in a 20-gallon tank. Neon tetras are small, peaceful schooling fish that can coexist well with molly fish.

3. Molly Fish And Zebra Danios

A combination of 4 molly fish and a school of 6-8 zebra danios could work in a 20-gallon tank.

Zebra danios are active swimmers and will utilize the upper level of the tank, complementing the molly fish.

4. Molly Fish And Platy Fish

In a 20-gallon setup, you can have around 4 molly fish and 4-5 platy fish. Both species are livebearers with similar care needs, making them compatible tankmates.

5. Molly Fish And Betta Fish

Keeping 3-4 molly fish with one betta fish in a 20-gallon tank is possible, but it requires close observation. Bettas can sometimes be territorial, so monitoring for aggression is important.


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

  • For a 20-gallon tank, it is recommended to keep four to five adult molly fish, considering the 1-inch per gallon rule.
  • A 20-gallon tank provides sufficient space for the natural behavior and growth of molly fish.
  • Overstocking the tank with molly fish can lead to poor water quality, increased disease risk, and aggression among the fish.
  • Managing the gender ratio, favoring more females to males, helps control the population growth of molly fish in a 20-gallon tank.
  • It is possible to breed molly fish in a 20-gallon setup, but specific conditions and careful management are necessary for successful breeding.