How Many Molly Fish In A 55-Gallon Tank? (With Examples)

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Getting a 55-gallon tank is a huge investment, and you probably want to do that right. I don’t blame you, I felt the same a few years back.

Since molly fish are one of the most common freshwater aquarium fish, the obvious question is how many of those you can keep in 55 gallons.

In this article, I will answer this question, and also discuss whether this tank size is suitable for breeding molly fish, how to avoid overcrowding, and what other fish species can be added as well.

Let’s get started.

How Many Molly Fish Can You Keep In 55 Gallons?

In a 55-gallon aquarium, you can comfortably keep between 12 to 13 adult molly fish.

These fish prefer company and ample space for swimming, which this tank size adequately provides.

The general rule of thumb, providing one gallon of water per inch of full-grown fish, is applicable here.

How Do I Determine The Appropriate Number Of Molly Fish For A 55-Gallon Tank?

You can calculate the number of mollies suitable for 55 gallons by following the one-inch-per-gallon rule:

  • Identify the adult size range of molly fish: 4 to 5 inches.
  • Determine the number of molly fish that can be accommodated in the tank based on the 55-gallon capacity: 55 inches (55 gallons x 1 inch/gallon).
  • Calculate the average size of the adult molly fish: (4 inches + 5 inches) ÷ 2 = 4.5 inches.
  • Divide the total tank capacity by the average adult size of the molly fish to find the maximum number of molly fish that can be housed in the tank: 55 inches ÷ 4.5 inches = 12.22 fish.
  • Since you can have a fraction of fish, the range for the number of molly fish that can be kept in a 55-gallon tank is between 12 to 13.

Why Is A 55-Gallon Tank Suitable For Molly Fish?

A 55-gallon tank offers a spacious environment for molly fish to exhibit natural behaviors and flourish.

The size of this tank aids in maintaining a balanced ecosystem that can greatly enhance the health and wellness of molly fish.

Here’s why this is the case:

  • Perfect Size: Since each molly fish needs between 5-10 gallons of water, a 55-gallon tank can conveniently house 5-11 molly fish.
  • Stable Conditions: A tank of 55-gallons provides a larger water volume that aids in keeping the water conditions stable, vital for the health of molly fish.
  • Room to Move: Being active swimmers, molly fish appreciate the large swimming and exploration space a 55-gallon tank provides.
  • Social Engagement: Molly fish can comfortably socialize without feeling overcrowded in a spacious 55-gallon tank, enhancing their living experience.
  • Replicating Nature: A 55-gallon tank allows for effective replication of molly fish’s natural habitats, complete with plants and hiding areas.

Also Read: Molly Fish Tank Size

Maintenance Tips For A 55-Gallon Tank With Molly Fish

To maintain a 55-gallon tank with molly fish, it is important to perform regular water replacements, monitor water parameters, provide a varied diet, and keep the tank clean.

Here are some key tips:

  • Scheduled Water Replacements: Replace 10-20% of the tank water every week to maintain cleanliness for molly fish.
  • Water Condition Monitoring: Keep a check on the pH, temperature, and nitrate levels in the tank regularly to ensure a suitable environment for molly fish. For that, I personally use the API FRESHWATER MASTER TEST (link to Amazon).
  • Nutritious Food: Serve a mixed diet of flakes, pellets, and occasional live or frozen food to your molly fish.
  • Regular Tank Cleaning: Clean the tank and filter routinely to remove waste and inhibit the growth of harmful bacteria.
  • Aquatic Plant Upkeep: Trim plants when needed to avoid overcrowding and ensure enough swimming space for molly fish.
  • Disease Awareness: Keep an eye on your molly fish for any signs of disease like unusual swimming, appetite loss, or color changes.

What Happens If You Add Too Many Molly Fish To A 55-Gallon Tank?

Overcrowding a 55-gallon tank with molly fish can lead to stress, disease spread, and unstable water conditions. Here’s why:

  • Stress: Overcrowding can cause stress among molly fish due to reduced personal space and increased competition for resources.
  • Disease Spread: A crowded tank facilitates the rapid spread of diseases among molly fish, increasing mortality rates.
  • Unstable Water Conditions: A higher number of molly fish means more waste, leading to rapid changes in water parameters.
  • Competition for Food: Too many molly fish can lead to competition for food, resulting in malnutrition for some fish.
  • Reduced Oxygen Levels: Overcrowding reduces the oxygen levels in the water, which can cause respiratory stress for molly fish.

Also Read: How Many Molly Fish In A 40-Gallon Tank?

Does The Gender Of Molly Fish Matter In A 55-Gallon Tank Setup?

No, the gender of molly fish doesn’t significantly affect a 55-gallon tank setup as both genders have similar needs and behavior. Here’s why:

  • Similar Environmental Needs: Both male and female molly fish require similar water conditions, diet, and space.
  • Comparable Behavior: Both genders are peaceful, making them compatible tank mates irrespective of the gender ratio.
  • Breeding: Since molly fish are livebearers, having both genders can lead to breeding, which can be managed in a 55-gallon tank setup.

Can Molly Fish Be Successfully Bred In A 55-Gallon Tank?

Yes, molly fish can be successfully bred in a 55-gallon tank due to the ample space and controlled environment. Here’s why:

  • Ideal Space: A 55-gallon tank provides enough space for breeding pairs and their offspring.
  • Controlled Environment: Breeders can control water parameters, diet, and lighting, crucial for successful molly fish breeding.
  • Separate Breeding Areas: The tank size allows for the setup of separate breeding areas, providing privacy for molly fish.
  • Safe Space for Fry: Post-breeding, the large tank allows for a separate, safe space for fry to grow.
  • Stress Reduction: A spacious tank reduces stress, which can increase breeding success in molly fish.

Is It Possible To Add More Molly Fish To A 55-Gallon Tank?

Yes, you can add more molly fish to a 55-gallon tank if the tank’s bioload allows it, and the added fish are introduced gradually to prevent stress. Here’s how:

  • Monitor Biodload: Ensure the tank’s biological load can handle the additional molly fish without impacting water quality.
  • Quarantine New Fish: Quarantine new molly fish before introducing them to prevent disease spread in the existing population.
  • Gradual Introduction: Add new molly fish gradually to prevent sudden changes in water parameters and reduce stress.
  • Monitor Social Dynamics: Observe the tank after adding new fish to ensure peaceful cohabitation among all molly fish.
  • Consider Compatibility: Make sure the added molly fish are compatible in terms of size, temperament, and dietary needs.

How Many Molly Fish Should Be Kept Together In General?

In general, it is often advised to keep a group of 4-5 Molly fish together to foster social interaction and maintain a harmonious community.

This group size facilitates natural behaviors like schooling and mating, while also preventing overcrowding and minimizing competition for resources.

Fish Species You Can Keep With Molly Fish In A 55-Gallon Tank

If you own a 55-gallon tank, you can easily mix other species with molly fish. Here is a simple table showing some quantities of common fish species:

Fish SpeciesNumber of Fish
Molly Fish (Alone)12-13
Molly Fish & Guppies10-12 Mollies & 10-12 Guppies
Molly Fish & Neon Tetras10-12 Mollies & 15-20 Neon Tetras
Molly Fish & Zebra Danios10-12 Mollies & 10-12 Zebra Danios
Molly Fish & Platy Fish10-12 Mollies & 10-12 Platy Fish
Molly Fish & Betta Fish10-12 Mollies & 1 Betta Fish

1. Molly Fish And Guppies

In a 55-gallon tank, you can keep about 10-12 molly fish along with around 10-12 guppies, given their peaceful nature and small size.

2. Molly Fish And Neon Tetras

For this combination, you could comfortably house around 10-12 molly fish and about 15-20 neon tetras. Neon tetras are small, schooling fish that take up minimal space.

3. Molly Fish And Zebra Danios

This setup could accommodate around 10-12 molly fish and approximately 10-12 zebra danios.

Zebra danios are active swimmers and cohabit well with mollies due to their peaceful nature.

4. Molly Fish And Platy Fish

In this configuration, you could keep 10-12 molly fish and about 10-12 platy fish. Platies have similar care needs and temperament to molly fish, making them ideal tank mates.

5. Molly Fish And Betta Fish

For this pairing, you could keep 10-12 molly fish and 1 betta fish.

Betta fish are territorial and typically, only one should be kept in a tank to prevent aggression. However, they generally coexist peacefully with molly fish.


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

  • In a 55-gallon tank, you can comfortably keep 12 to 13 adult molly fish, providing adequate space for swimming and social interaction.
  • A 55-gallon tank offers a spacious and balanced ecosystem for molly fish, promoting their health and well-being.
  • To maintain a 55-gallon tank with molly fish, regular water replacements, monitoring of water parameters, varied diet, and tank cleaning are important.
  • Overcrowding a 55-gallon tank with molly fish can lead to stress, disease spread, unstable water conditions, and competition for food.
  • The gender of molly fish doesn’t significantly affect a 55-gallon tank setup, and molly fish can be successfully bred in this tank size with controlled conditions.