How Many Mollies Should Be Kept Together? - "male alpha molly" by h080 is licensed under CC BY-SA 2.0.
How Many Mollies Should Be Kept Together? - "male alpha molly" by h080 is licensed under CC BY-SA 2.0.


How many mollies should be kept together? This question often arises among aquarium enthusiasts as they embark on the journey of creating a vibrant and thriving aquatic community. Mollies, with their lively personalities and striking colors, are a popular choice for many hobbyists. However, understanding the ideal social dynamics and tank requirements for these fish is crucial to ensure their well-being and promote a harmonious environment. In this guide, we will explore the factors to consider when setting up a tank for mollies, including tank size, gender ratios, how many should be kept together, and other essential aspects, to provide the best possible care for these captivating aquatic companions.

How Many Mollies Should Be Kept Together in Different Tank Sizes?

The number of mollies that can be kept together in an aquarium depends on various factors, including the size of the tank, the gender mix of the mollies, and the overall setup of the aquarium. Mollies are social fish, and it’s generally recommended to keep them in groups to ensure their well-being and reduce stress.

Here are some guidelines for different tank sizes:

10-Gallon (40 liter) Tank

How many mollies should be kept together in a 10 gallon (40 liter) tank? For a 10-gallon (40 liter) tank, it’s best to keep a small group of mollies, such as 2 to 4 individuals. Since mollies can grow quite large (up to around 4 inches – 10 centimeters), a 10-gallon (40 liter) tank is suitable only for a small number of mollies. Keeping too many in a small tank can lead to overcrowding and water quality issues.

20-Gallon (80 liter) Tank

With a 20-gallon (80 liter) tank, you can keep a slightly larger group of mollies. Aim for a group of 4 to 6 mollies, depending on the gender mix. If you have a mix of males and females, ensure that there are more females than males to reduce aggression.

30-Gallon (120 liter) Tank

In a 30-gallon (120 liter) tank, you can have a slightly larger community of mollies. A group of 6 to 10 mollies would be suitable for this size of the tank. Again, keep an eye on the gender ratio to avoid excessive male aggression.

Remember, mollies are active fish, and they produce waste, which can affect water quality. Regular water changes and good filtration are essential regardless of tank size.

Also, it’s essential to provide plenty of hiding places and plants in the aquarium to create a stress-free environment for the mollies.

Always research the specific needs of the mollies you plan to keep and monitor their behavior and well-being regularly. If you notice any signs of stress or aggression, consider adjusting the group size or setup accordingly.

Gender Ratio

The gender ratio for keeping mollies in an aquarium is crucial to maintain a harmonious environment and prevent aggressive behavior, also when you’re considering how many mollies should be kept together. Mollies can be somewhat territorial and aggressive, especially males, so getting the right gender balance is essential.

The ideal gender ratio for mollies is to have more females than males. A recommended ratio is approximately 1 male to 2 to 3 females. This will help reduce the aggression between male mollies and spread out any potential aggression across multiple females.

The reason for this ratio is that male mollies can be insisting when it comes to mating and may constantly chase and harass a single female if there are too few females. With multiple females, the male’s attention is divided, reducing the stress on any one individual.

To maintain a balanced and healthy molly community, follow this guideline for the gender ratio:

  • 1 male molly : 2 to 3 female mollies

This ratio, combined with adequate hiding places and plants in the aquarium, will create a more stable and peaceful environment for your mollies. Always monitor their behavior and make adjustments as needed if any signs of aggression or stress are observed.

How Many Mollies Should Be Kept Together? - "male alpha molly" by h080 is licensed under CC BY-SA 2.0.
How Many Mollies Should Be Kept Together? – “male alpha molly” by h080 is licensed under CC BY-SA 2.0.

Tank Setup

Another important aspect when considering how many mollies should be kept together, is setting up the fish tank right.

Setting up the tank for mollies involves creating a suitable environment that meets their specific needs for water quality, temperature, decorations, and space. Here’s a step-by-step guide on how to set up a tank for mollies:

  • Selecting the Tank: Choose an appropriately sized tank for the number of mollies you plan to keep. As discussed earlier, a 20 to 30-gallon tank is ideal for a small community of mollies. Ensure the tank has a sturdy stand to support its weight when filled with water.
  • Water Quality and Filtration: Use a good-quality water filter to maintain water clarity and remove impurities.
  • Heating and Temperature: Mollies are tropical fish and need a stable water temperature between 75°F to 82°F (24°C to 28°C). Use a reliable aquarium heater to maintain the desired temperature.
  • Substrate: Choose a fine-grained substrate like sand or smooth gravel. Avoid rough or sharp substrates that may harm the mollies’ delicate fins and mouths.
  • Decorations and Hiding Places: Provide plenty of hiding spots and decorations in the tank. Mollies like to explore and may seek shelter when they feel stressed. Add live or artificial plants to provide cover and create a more natural environment. Caves, driftwood, and rocks can also be used for hiding places.
  • Lighting: Provide moderate lighting in the tank. Mollies don’t require intense lighting, but some light is necessary for their well-being and to support any live plants.
  • Cycling the Tank: Before introducing mollies, cycle the aquarium to establish beneficial bacteria that help break down toxic ammonia and nitrite.
  • Adding Mollies: Once the tank is cycled and water parameters are stable, it’s time to add the mollies. Acclimate them to the new environment by floating the bag in the tank and gradually adding small amounts of tank water to the bag over a period of time. This helps them adjust to the water temperature.
  • Feeding: Offer a balanced diet of high-quality flake or pellet food specifically formulated for mollies. Supplement their diet with occasional treats like frozen or live foods such as brine shrimp or daphnia.
  • Water Maintenance: Perform regular water changes (about 20-25% every week) to keep water quality in check and remove accumulated waste. Clean the filter regularly as well.
  • Observation: Observe your mollies regularly to ensure they are behaving normally and showing no signs of stress or illness. If any issues are noticed, take appropriate action promptly.

Remember, mollies are social fish, so it’s best to keep them in groups as discussed earlier. Providing a well-maintained and enriched environment will promote the health and well-being of your mollies, making for happy and vibrant fish in your aquarium.


In conclusion, creating an optimal environment for mollies is essential to ensure their well-being and promote a harmonious aquarium community. How many mollies should be kept together depends on factors such as tank size and gender ratio. For a 20 to 30-gallon (80-120 liter) tank, it is recommended to keep a small community of mollies, maintaining a gender ratio of approximately 1 male to 2 to 3 females. By following these guidelines and providing suitable water conditions, decorations, and regular care, aquarists can enjoy the beauty and social dynamics of mollies as they thrive in their enriched aquatic habitat. How many mollies should be kept together? The answer lies in creating a balanced and nurturing environment that meets the specific needs of these charming fish.