Molly Fish Tank Size: A Complete Beginner’s Guide

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Molly fish are one of the most common freshwater fish in home aquariums and for a good reason. They are extremely beautiful and get along with many other species.

However, these creatures also raise many questions, with one of the most common being their ideal tank size.

In this article, I will discuss the minimum and best tank sizes for mollies and also show you how many you can keep in 5, 10, 20, 30, 40, and 55 gallons.

Let’s dive right in.

Smallest Acceptable Aquarium Size For Molly Fish

My recommendation: Aqueon 20 Gallon Starter Kit (link to Amazon).

Molly fish need at least a 20-gallon aquarium. This size is vital to ensure their good health, wellbeing, and colorful lifestyle.

Let’s explore why a 20-gallon aquarium is essential for molly fish, focusing on these aspects:

  • Room to Swim: Molly fish are active swimmers who need plenty of room. A 20-gallon tank allows them to roam freely, reducing stress and encouraging natural behavior.
  • Accommodating Growth: Mollies can grow up to 4.5 inches. A 20-gallon tank caters to their growth, preventing health problems caused by cramped spaces.
  • Water Stability: Larger tanks tend to have more stable water conditions. With a 20-gallon tank, the risk of rapid water parameter changes is minimized, offering a safer habitat for molly fish.
  • Behavioral Expression: A spacious 20-gallon tank lets molly fish show off their natural behaviors, improving their psychological health.
  • Aquascaping Space: A 20-gallon tank has plenty of room for aquascaping, providing molly fish with hiding spots and areas to explore, enhancing their environment.

Best Aquarium Size For Molly Fish

While a 20-gallon aquarium is the smallest suitable size for molly fish, a 30-gallon tank, or larger, is ideal.

Such a size offers plenty of room for the molly fish to roam, grow, and thrive, improving their overall health and wellbeing.

Let’s break down why a 30-gallon or larger tank is preferable:

  • More Swimming Room: A larger tank gives molly fish extra space for swimming. This promotes their physical health and matches their natural mobility habits.
  • Stable Water Conditions: Bigger tanks maintain stable water parameters. A 30-gallon or larger tank reduces the likelihood of rapid water quality changes, promoting molly fish health.
  • Behavioral Freedom: A larger space lets molly fish display their natural behaviors more effectively. A 30-gallon tank gives them more room to interact, improving their mental wellbeing.
  • Aquascaping Options: A bigger tank allows for more elaborate aquascaping. This gives molly fish more places to hide and explore, enriching their surroundings.
  • Waste Dilution: Larger tanks dilute waste products more effectively. This lowers the concentration of harmful substances, making a healthier environment for molly fish.

Also Read: Molly Fish Tank Setup

How Many Molly Fish Can Fit In An Aquarium?

The number of molly fish you can fit in an aquarium depends on its size. Here is a short breakdown when considering 5, 10, 20, 30, 40, and 55 gallons:

Tank SizeNumber of Molly Fish
5-Gallon TankNot suitable
10-Gallon Tank2 to 3
20-Gallon Tank4 to 5
30-Gallon Tank6 to 7
40-Gallon Tank8 to 10
55-Gallon Tank12 to 13

5-Gallon Aquarium

Generally, a 5-gallon tank is not suitable for Molly fish due to their active nature and size. Let’s explore why this is the case:

  • Space requirement: Molly fish require ample space for swimming, so a 5-gallon tank is typically too small for these active creatures.
  • Health impact: Inadequate space can lead to stress and diseases in molly fish, impacting their overall lifespan and well-being.
  • Water quality: A smaller tank can’t dilute waste effectively, resulting in poor water quality which can harm the molly fish.

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

10-Gallon Aquarium

A 10-gallon aquarium can accommodate 2 to 3 molly fish, but this is dependent on the species and other tank conditions. Here are some points to keep in mind:

  • Species size: Some molly fish are smaller and may be comfortable in a 10-gallon tank, but larger species will feel cramped.
  • Tank mates: If there are other fish species in the tank, it might create territorial disputes or overpopulate the aquarium.
  • Tank environment: Molly fish thrive with plenty of hiding spots and plant life, which can be challenging to accommodate in a 10-gallon tank.

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

20-Gallon Aquarium

A 20-gallon aquarium offers more comfort for around 4 to 5 molly fish. Let’s delve into some supporting facts:

  • Comfortable space: A 20-gallon tank provides enough room for molly fish to swim and explore, thus reducing stress levels.
  • Better water quality: Larger tanks dilute waste more effectively, promoting a healthier environment for molly fish.
  • Varied species accommodation: This size allows for a diversity of molly fish species, considering their different sizes and temperaments.

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

30-Gallon Aquarium

A 30-gallon aquarium can host approximately 6 to 7 molly fish comfortably. Here’s why:

  • Ample space: More room means less competition for territory among molly fish, reducing the chance of aggressive behaviors.
  • Improved tank environment: A larger tank can accommodate more plants and hiding spots, which molly fish appreciate.
  • Population control: With more space, it’s easier to manage molly fish breeding, which can get out of control in smaller tanks.

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

40-Gallon Aquarium

About 8 to 10 molly fish can thrive in a 40-gallon aquarium. Let’s take a look at the reasons:

  • Spacious swimming area: A 40-gallon tank allows molly fish to swim freely, improving their overall well-being.
  • Diverse tank mates: This size permits a greater variety of species, including other compatible fish types alongside molly fish.
  • Easier maintenance: Larger tanks tend to maintain more stable water conditions, which benefits the health of molly fish.

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

55-Gallon Aquarium

A 55-gallon tank can house up to 12-13 molly fish comfortably. Here’s why:

  • Generous space: A 55-gallon aquarium offers an abundance of room for molly fish, reducing territorial disputes and stress.
  • Improved tank conditions: Larger tanks offer better water quality and a more stable environment for molly fish.
  • Species variety: Such a tank can comfortably house different molly fish species, enhancing the visual appeal and diversity of your aquarium.

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

What Happens If You Add Too Many Molly Fish

Overpopulating a tank with molly fish can cause problems, affecting their health and survival:

  • Stress and Aggression: Overcrowding can increase stress levels and aggression in molly fish, possibly leading to injuries or even death.
  • Water Quality: Overpopulation speeds up waste production, resulting in a swift decline in water quality, which can harm molly fish.
  • Disease Risk: In crowded tanks, disease can spread quickly due to close quarters, potentially leading to widespread sickness or death.
  • Food Competition: More fish mean more competition for food. Some fish may not get enough to eat, leading to malnutrition or stunted growth.
  • Limited Space: Overpopulation reduces the available space for each fish, which can stunt growth and shorten life expectancy due to constant stress and poor living conditions.

Also Read: Do Molly Fish Need Light?

Can You Keep Molly Fish In A Glass Bowl?

A glass bowl is not a suitable environment for molly fish.

The confined space and subpar water quality associated with bowls can stress molly fish and reduce their overall health and lifespan.

Here’s why a glass bowl is unsuitable:

  • Limited Swim Space: Molly fish need room to swim. A small glass bowl doesn’t provide the necessary room for comfortable movement and active swimming.
  • Poor Water Quality: It’s hard to maintain stable water parameters in a glass bowl. Rapid water quality changes can stress molly fish and cause health issues.
  • Lack of Filtration: Glass bowls usually don’t have good filtration. Without proper filtration, waste builds up fast, creating a toxic environment for molly fish.
  • Limited Social Interaction: Molly fish are social and thrive in groups. A glass bowl is too small for multiple molly fish, limiting their social interactions.
  • Stunted Growth: The confined space in a glass bowl can stunt the growth of molly fish. Stunted growth is harmful and can reduce their lifespan.

Also Read: Do Molly Fish Need A Heater?


The smallest tank size for a molly fish is 20 gallons. Keeping mollies in tanks smaller than that will cause stress and overcrowding.

That is primarily because mollies are relatively large species that can grow to a size of 4.5 inches, and therefore, require more space.

Ideally, if you can, aim for 30 gallons or more. The larger the tank, the easier it will be to maintain high water quality and stress-free molly fish.