How Many Molly Fish Per Gallon: The Quick Answer

Disclosure: When you purchase something through my affiliate links, I earn a small commission. As an Amazon Associate, I earn from qualifying purchases.

If you wish to grow molly fish but don’t know how many to pick, you have come to the right place.

In this article, I will discuss how many mollies should be kept per gallon, and how many should accommodate 5, 10, 20, 30, 40, and 55 gallons.

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

How Many Molly Fish Per Gallon?

The recommended number of molly fish per gallon is typically one fish for every 2-3 gallons of water.

This guideline is crucial in order to ensure the health and well-being of your molly fish. Here are a few points to consider to better understand this requirement:

  • Space requirements: Molly fish need adequate space to swim and interact; thus, keeping them in overcrowded conditions can cause stress and increase the risk of disease.
  • Molly Fish size: Adult molly fish can grow up to 4.5 inches in size, further necessitating the need for ample space per fish.
  • Water quality: Molly fish require clean, well-filtered water. Overcrowding can lead to a rapid decline in water quality, affecting the health of the molly fish.
  • Social behavior: Molly fish are social creatures and thrive in groups. However, too many fish in a small area can cause conflict and stress among them.
  • Breeders’ recommendation: Most breeders and experts suggest that providing molly fish with sufficient space promotes their overall health and longevity, thereby improving their quality of life.

Also Read: 15 Things You Should Know About Molly Fish

How Many Molly Fish In A 5-Gallon Tank?

A 5-gallon tank isn’t really spacious enough for molly fish. These aquatic creatures thrive better with more room to swim and socialize.

Stuffing them into such a limited space could compromise water cleanliness and ramp up the fish’s stress levels.

Also Read: How Many Molly Fish In A 5-Gallon Tank?

How Many Molly Fish In A 10-Gallon Tank?

A 10-gallon tank can comfortably play host to 2-3 molly fish.

This amount ensures each molly fish can enjoy sufficient room to swim, mature, and mingle with other tank inhabitants.

It’s also beneficial for preserving water hygiene and curbing the spread of diseases.

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

How Many Molly Fish In A 20-Gallon Tank?

A 20-gallon tank is a good fit for about 5 molly fish. With this volume, the tank offers ample room for the molly fish to swim at will and engage in their social antics.

It also promotes better regulation of water purity, which is vital for the molly fish’s health.

Also Read: How Many Molly Fish In A 20-Gallon Tank?

How Many Molly Fish In A 30-Gallon Tank?

A 30-gallon tank is suitable for roughly 6-7 molly fish.

This provides each fish with abundant room to grow, lowering the chance of disputes or stress linked to overcrowding.

Besides, a tank of this size allows for ideal water quality management.

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

How Many Molly Fish In A 40-Gallon Tank?

A 40-gallon tank can comfortably hold 8-10 molly fish.

This grants the molly fish an opportunity to display their natural behaviors, including social bonding and exploration.

It also aids in ensuring that water purity is kept in check, essential for the fishes’ health and wellness.

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

How Many Molly Fish In A 55-Gallon Tank?

A 55-gallon tank is capacious enough for about 12-13 molly fish. This setup offers each molly fish plenty of room for swimming, growth, and social interaction.

It also aids in sustaining water quality at peak levels, which is imperative for the molly fish’s health.

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

Why Is It Important To Determine The Number Of Molly Fish Per Gallon?

Choosing the right number of mollies for your tank is necessary for several reasons:

1. Avoid Overcrowding

Molly fish, like all aquatic creatures, require adequate space to live comfortably and healthily. Overcrowding the tank can lead to stress, disease, and death in molly fish.

Here are the reasons why overcrowding must be avoided:

  • Stress Reduction: A crowded tank can cause molly fish to become stressed, leading to a weakened immune system and susceptibility to disease.
  • Disease Prevention: Overcrowded conditions often lead to poor water quality, which can facilitate the spread of diseases among molly fish.
  • Territorial Disputes: Molly fish can become territorial, especially in crowded conditions, causing unnecessary aggression and potential harm to the fish.
  • Proper Growth: Providing the right amount of space per fish allows each molly fish to grow to its full potential.
  • Behavior Observation: With adequate space, molly fish display natural behaviors, which is not only better for their well-being but also more entertaining for observers.

2. Simplify Tank Maintenance

The number of molly fish per gallon also directly influences the ease of maintaining the tank. A tank with too many fish can be difficult to clean and keep at optimal conditions.

Here are some reasons why proper stocking simplifies maintenance:

  • Reduced Waste: Fewer molly fish means less waste produced, resulting in a cleaner tank and less frequent cleaning necessary.
  • Stable Water Parameters: Maintaining the right number of molly fish helps keep the water chemistry stable, which is crucial for their health.
  • Easy Monitoring: With an appropriately stocked tank, it’s easier to monitor each molly fish for signs of stress, disease, or abnormal behavior.
  • Lower Oxygen Demand: Fewer molly fish result in lower oxygen demand, reducing the risk of oxygen depletion.
  • Less Algae Growth: Overstocked tanks often have excess nutrients, leading to unwanted algae blooms. This can be avoided with the correct number of molly fish.

3. Allocate Space And Resources For New Fish

Accommodating new molly fish is a common aspect of keeping an aquarium, and ensuring there’s enough room is crucial.

This applies both to new fish purchases and to mollies’ prolific breeding habits. Here are the reasons why proper space allocation matters:

  • Healthy Breeding: Molly fish are livebearers, and the females can give birth to a large number of fry. Adequate space is necessary for the survival and growth of these baby fish.
  • Adding Diversity: Properly spaced molly fish allows for the introduction of different species, adding diversity to the tank.
  • Quarantine Zone: Providing enough space allows for isolation zones, which are important for new molly fish introductions or when treating sick fish.
  • Avoiding Competition: Proper space allocation reduces competition for resources among the molly fish, promoting peaceful cohabitation.
  • Controlled Population: By controlling the number of molly fish per gallon, you can manage the population growth, which is important given mollies’ high reproduction rate.

Different Calculation Approaches

As you might have already guessed, there are several ways to calculate the right number of mollies for your tank. Here are the main ones:

1. Calculation Techniques Based On General Guidelines

General guidelines provide an easy starting point for determining the number of molly fish per gallon.

These guidelines are widely used and allow beginners to easily start their aquarium hobby. Here’s how these general guidelines work:

  • Inch per Gallon Rule: This rule suggests 1 gallon of water per inch of molly fish. However, it’s a very simplistic rule and doesn’t account for fish size, shape, or behavior.
  • 20 Percent Rule: This rule recommends reducing the Inch per Gallon rule by 20% to account for potential growth of molly fish and other factors.
  • Aquarium Type: The type of aquarium (freshwater, brackish, saltwater) also influences these guidelines, as different types of molly fish thrive better in different conditions.

2. Calculation Techniques Based On Fish Size

The size of molly fish plays a crucial role in determining the number of fish per gallon. Larger fish obviously require more space, and molly fish can grow to be 4.5 inches long on average.

Here’s how fish size impacts the calculation:

  • Fish Length: The length of molly fish, which can reach up to 4.5 inches, is a major determinant of the required tank size.
  • Fish Shape: Molly fish have a taller body shape compared to many fish species. This means they need more water height, not just floor space.
  • Growth Potential: The potential for molly fish to grow should also be accounted for when stocking a tank, especially in the case of juvenile fish.

3. Calculation Techniques Based On Tank Capacity

Tank capacity is another key factor for determining the number of molly fish per gallon. The size and shape of the tank have significant impact on the molly fish’s health and wellbeing.

Here’s how tank capacity affects the calculation:

  • Tank Size: The size of the tank directly limits the number of molly fish that can live in it comfortably. A larger tank generally means more fish.
  • Tank Shape: A taller tank will hold more water but offers less surface area for oxygen exchange, affecting the number of molly fish it can support.
  • Filtration Capacity: The tank’s filtration capacity also plays a crucial role, as it needs to efficiently process the waste of all molly fish present.

What I Recommend

I personally prefer the tank capacity-based calculation method because it considers multiple variables, including the equipment used in the aquarium.

This approach gives a more complete picture of the aquarium’s conditions and better caters to the unique requirements of molly fish.

Simply follow the general rules of how many mollies can be kept in 5, 10, 20, 30, 40, and 55 gallons, as I discussed earlier.

Also Read: How Long Do Molly Fish Live?

Do Female Molly Fish Require More Gallons Than Males?

No, female molly fish do not necessarily require more gallons than males. Both male and female molly fish have similar space requirements based on their size and biological needs.

However, if breeding is planned, extra space might be beneficial to accommodate the female molly’s offspring and reduce potential aggression or stress.

Also Read: Male vs. Female Molly Fish


For those of you in a rush, here is a quick overview of what I discussed earlier:

  • Overcrowding molly fish in a tank can lead to stress, disease, and territorial disputes, affecting their overall health and well-being.
  • Tank maintenance becomes simpler with the right number of molly fish, as it reduces waste production, stabilizes water parameters, and lowers oxygen demand.
  • Properly allocating space and resources for molly fish is important for healthy breeding, introducing new species, and managing population growth.
  • Different calculation approaches, such as general guidelines, fish size, and tank capacity, can be used to determine the ideal number of molly fish per gallon.
  • Female molly fish do not require more gallons than males, but providing extra space might be beneficial if breeding is planned to accommodate their offspring and minimize aggression or stress.