How Many Molly Fish In A 5-Gallon Tank? (Mistakes To Avoid)

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Many aquarists start their fishkeeping journey with a 5-gallon tank. However, you should know from the start that this tank size has its limitations.

In this article, I will discuss the number of molly fish that can be kept in such a relatively small tank and provide tips for maintaining their health.

Additionally, I will discuss the compatibility of other fish types that can coexist with mollies in a 5-gallon tank, such as neon tetras, guppies, zebra danios, platy fish, and bettas.

Let’s get started.

How Many Molly Fish Can You Keep In A 5-Gallon Tank?

Typically, a 5-gallon tank is inadequate for mollies as they require sufficient swimming area and room for growth.

However, if it’s absolutely necessary, you can technically house one small molly fish in it. Bear in mind that this arrangement is not ideal and can negatively affect the fish’s health.

How Do You Determine The Number Of Molly Fish Suitable For A 5-Gallon Tank?

Here is how to calculate the number of molly fish that can theoretically be kept in a 5-gallon tank, based on the one-inch-per-gallon rule:

  • Determine the adult size of the molly Fish, which usually averages between 3-4 inches in length.
  • You can apply the one-inch-per-gallon rule, which recommends allocating one gallon of water for every inch of fish.
  • Multiply the tank size by the one-inch-per-gallon rule. In a 5-gallon tank, you would theoretically be able to house 5 inches of fish.
  • Divide the total inches of fish the tank can hold by the size of the fish. If molly fish are 3 inches each, a 5-gallon tank can hold just over 1 molly Fish.
  • Round down to the nearest whole number for fish count. So in this case, a 5-gallon tank can hold 1 molly Fish.

What Is The Recommended Tank Size For Molly Fish?

Molly fish, known for their active swimming behavior and growth in size, need ample space to thrive.

It is generally recommended to provide them with a minimum tank size of 20 gallons, which allows for adequate swimming room and helps maintain stable water conditions.

However, a 30-gallon tank is considered more ideal as it offers even more space, promoting better health and well-being.

Also Read: Molly Fish Tank Size

Why 5 Gallons Might Not Provide Enough Space

A 5-gallon tank is generally deemed insufficient for molly fish due to their behavioral and physiological needs.

While the one-inch-per-gallon guideline serves as a basic reference, there are crucial factors that make a 5-gallon tank unsuitable:

  • Molly Fish Growth and Size: Mollies typically grow to 3-5 inches in adulthood, and confinement in small spaces can induce health problems and hinder their growth.
  • Activity Level of Molly Fish: As active swimmers, molly fish require ample space to move around. A 5-gallon tank might not provide enough space for them to express natural behaviors, leading to potential stress.
  • Bio-load Considerations: Molly fish produce a significant amount of waste. A 5-gallon tank may have difficulty maintaining the optimal water quality necessary, even with frequent cleaning.
  • Social Requirements: Molly fish are gregarious and often prefer group living. A 5-gallon tank lacks sufficient space to house a group of mollies, leading to potential stress and territorial conflicts.
  • Environmental Diversity: Larger tanks can accommodate a more diverse environment, including plants and hiding spots, enhancing the molly fish’s overall wellbeing.

Tips For Growing Molly Fish In A 5-Gallon Tank

Although it can be challenging, it is possible to keep a single molly fish in a 5-gallon tank by adhering to the following guidelines:

  • Ensure Water Quality: To maintain a healthy environment for your molly fish, change the water frequently, ideally 25-50% every week.
  • Use a Filter: Implementing an efficient filter is crucial to cope with the molly fish’s bio-load and to maintain stable water parameters.
  • Track Water Parameters: Consistently monitor the pH, temperature, and ammonia levels. Molly fish thrive in slightly alkaline water with a pH of 7.5-8.5 and temperatures between 72-78°F.
  • Provide a Balanced Diet: Feed your molly a varied diet that includes both commercial flake or pellet foods and occasional fresh, frozen, or freeze-dried treats to ensure it gets all the necessary nutrients.
  • Minimize Stress: Reduce stress by keeping the tank in a quiet area, minimizing disturbances, and gradually introducing any changes in the tank environment.
  • Add Plants and Decor: Despite the size restrictions, try to include some live plants and hiding spots for the molly fish to retreat to when it feels threatened.
  • Ensure Enough Swimming Space: Make sure decor and plants don’t overly restrict the swimming space, as mollies are active swimmers.

Does The Gender Of Molly Fish Matter?

No, the gender of molly fish does not significantly influence the decision on tank size.

Both male and female mollies require ample space for healthy growth and activity, regardless of their gender.

It is their adult size, behavior, and social needs that primarily determine the appropriate tank size.

Will Breeding Cause Overcrowding In A 5-Gallon Tank?

Yes, breeding molly fish in a 5-gallon tank could certainly lead to overcrowding.

Given that mollies can give birth to dozens of fry at a time, such a small tank would quickly become insufficient to sustain the growing population.

Is It Possible To Breed Molly Fish In A 5-Gallon Setup?

Technically, you could breed molly fish in a 5-gallon setup, but this is highly discouraged. 

The tank size is simply not sufficient to provide the necessary space for the adult molly fish and their offspring.

How Often Do Molly Fish Breed?

Molly fish are known for their high reproductive rate. They breed very often, typically once every 30 to 40 days.

Furthermore, during each breeding cycle, molly fish can give birth to a substantial number of fry, occasionally reaching as many as 100 offspring.

This frequent and substantial reproduction further highlights the need for larger tank sizes to comfortably accommodate the rapidly expanding population.

What Is The Ideal Number Of Molly Fish To Keep Together?

Maintaining a small group of 4-5 molly fish is generally considered ideal since these social fish thrive in the presence of their own kind.

Obviously, larger tanks can accommodate larger groups, as long as you ensure good water quality and provide sufficient space for swimming and exploration.

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

Can You Add More Molly Fish To A 5-Gallon Tank?

No, adding more molly fish to a 5-gallon tank is not recommended.

Given their adult size, active nature, and waste production, even a single molly fish can stretch the limits of a 5-gallon tank.

Adding more mollies would likely lead to overcrowding and poor water quality.

Can You Keep Mollies With Other Fish In A 5-Gallon Tank?

Unfortunately, since a 5-gallon tank is almost too small for a single molly fish, you can’t actually add other types of fish to live alongside your molly.

Here is a brief explanation for guppies, neon tetras, zebra danios, platy fish, and bettas:

1. Molly Fish And Guppies

A 5-gallon tank is not suitable for housing both molly fish and guppies, as both species are active and require larger tanks to live healthily.

A larger tank of at least 20 gallons would be a more appropriate size for this combination.

2. Molly Fish And Neon Tetras

Similarly, a 5-gallon tank is too small to accommodate both molly fish and neon tetras.

Both species are schooling fish and need ample swimming space. A 20-gallon tank is a better size for this combination.

3. Molly Fish And Zebra Danios

A combination of molly fish and zebra danios would not thrive in a 5-gallon tank due to their size and active nature. A 10-gallon tank, or even larger, would be a more suitable habitat.

4. Molly Fish And Platy Fish

A 5-gallon tank is not large enough to house both molly fish and platy fish.

Both species need more space than this tank size provides, with a minimum of 20 gallons recommended for their cohabitation.

5. Molly Fish And Betta Fish

Keeping a molly fish and a betta fish in a 5-gallon tank is not advisable.

Betta fish can be territorial and might not tolerate sharing such a small space with another fish, and mollies need larger spaces to swim.

A 10-gallon tank is the minimum recommended for a single betta fish, and a larger tank would be needed to add mollies.


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

  • A 5-gallon tank is insufficient for housing mollies due to their need for swimming space and growth.
  • Breeding molly fish in a 5-gallon tank can lead to overcrowding and is not recommended.
  • The recommended tank size for mollies is a minimum of 20 gallons, with 30 gallons considered more ideal.
  • Molly fish require slightly alkaline water with a pH of 7.5-8.5 and temperatures between 72-78°F.
  • Adding more mollies or other fish to a 5-gallon tank is not advisable, as it can result in overcrowding and poor water quality.