How Many Mollies Should Be Kept Together? 6 Important Factors to Consider

Loved for their hardiness, color varieties, and attractiveness, molly fish remain popular in many aquariums worldwide. If you love a shoaling fish, mollies are your perfect fit. However, one thing you may want to know is, “How many mollies should be kept together?” 

Three to four mollies should be kept together to form a small shoal. Mollies love and are comfortable sticking together. Staying together is a social instinct that helps them keep safe in their natural habitats. 

If you’ve been wondering how many mollies you should keep, this guide will help you know the number. You’ll also know how many mollies to keep in 20 and 10-gallon (75.71 and 37.85-liter) tanks. Keep reading!

How Many Mollies Should I Have In My Tank? 

You should have at least three to four mollies in your tank for an active and social molly fish community. A study conducted by Researchgate confirms that mollies gain significant benefits by living in shoals. 

Some benefits of keeping mollies together include:

  • Increased foraging success
  •  Improved safety 
  •  Mollies are more comfortable and active because they’re familiar with each other. 

That notwithstanding, the number of mollies in a tank depends on the tank size. Therefore, provide the correct tank capacity for the right number of mollies to promote comfort, fast growth, a healthy lifestyle, and peaceful socialization. 

The table below provides a general guideline for the tank sizes and how many mollies can be kept in them. 

Tank Size in GallonsTank Size in LitersMaximum Number of Mollies
Molly fish tank size chart

Overcrowding becomes a serious problem if the number of mollies exceeds tank size. 

An overcrowded tank has more cases of disease, cannibalism, and stunted growth. Fish in such an environment also compete for food and other resources, such as lighting and hiding spots while putting pressure on the filtration systems.

That’s definitely a poor environment that your mollies won’t relish. It’s a sign of poor care and management and a very dangerous state to expose your wet pets to.

How Many Mollies Should Be Kept Together in a 20-Gallon Tank? 

how many mollies should be kept together

At least five to six mollies can comfortably be kept in a 20-gallon (75.71-liter) tank. Generally, one inch (2.54 cm) of molly fish can occupy one gallon (3.79 liters) of water. Hence, six three-inch (7.62-cm) molly fish will fit in a 20-gallon (75.71-liter) tank. 

The following factors affect how many mollies you should keep in a 20-gallon (75.71-liter) tank: 

  • Molly fish size: You can raise even more mollies in a 20-gallon tank, depending on their size. Based on this information, you can have up to nine two-inch mollies living in a tank of that capacity. However, determining the number of mollies in a tank based on size takes work. Some molly fish grow faster than others. The result is a mix of small and large mollies in the same tank—this makes knowing the exact number of fish pretty hard. 
  • Swimming behavior: Remember, mollies are active swimmers and need plenty of space for roaming in the water. 
  • Tank mates: You may want a community of different fish in your aquarium. So, you bring in another fish type, like the bristlenose plecos. At this point, your molly fish number in a 20-gallon (75.71-liter) tank will be reduced to accommodate the tank mates. 
  • Decorations, rocks, and plants in the tank: You want to consider this when stocking mollies. As you can rightly imagine, decorations, rocks, and plants play vital roles in the ecosystem and would equally take up space as much as the fish.

How Many Mollies in a 10-Gallon Tank?

You can keep three to four mollies in a 10-gallon (37.85-liter) tank without having management issues. Of course, assuming that each one-molly-inch occupies one gallon of water. 

The number of mollies you keep in such a tank can vary, thanks to factors like molly breed, growth stage, tank mates, and other objects in the tank. Let’s discuss how these factors determine the number of mollies you keep in the tank.

1. Molly Breed

Sailfin molly

Considering the fish breed, sailfin mollies are larger than other types. Consequently, you may need to raise only two in a 10-gallon (37.85 liter) tank. The same applies to black mollies, which are also bigger.

Maybe two sailfin or black mollies in a 10-gallon (37.85-liter) tank will be okay because they need more space. 

You can have several small molly varieties in the tank without problems.

2. Growth Stage

You can have as many juveniles in the same tank as possible since they’re still small and occupy less space. However, that won’t be long until you change some of them to another tank because molly fish grow fast—they attain adult size at only 4 months.

Adult mollies will need more space. You can allocate them space depending on their size.

3. Number of Tank Mates

If you intend to have another fish species in the same tank, you must rethink the number of mollies. Mollies can live comfortably with many livebearer fish species like Rosy Barbs, Guppies, Swordtail, and Platy.

4. Presence of a Filtration System

Mollies grow well and are healthy when you have an efficient filter in the aquarium. The filter removes impurities to keep the water clean and fresh.

Don’t keep more than one fish in the tank if it doesn’t have a high-quality filter. This helps reduce the level of waste and other impurities.

Once you install a reliable filter, you can raise two or three fish in the 10-gallon (37.85-liter) tank and rest assured that all is well with the water quality.

5. Tank Setup

Your fish tank could have plants, decorations, and hiding spots—all designed to improve the fish’s welfare. 

However, these additions compromise the amount of space available for your mollies to swim and live a happy life.

For that reason, it’s appropriate to consider these mentioned objects when planning the number of mollies to keep.

6. Molly Fish Gender

In the case of keeping both male and female mollies in the same tank, you’re likely to have a busy, overcrowded tank all thanks to the fish’s breeding activities. Eventually, you’ll have the fry taking up their fair share of space.

It’s recommended to breed mollies in a separate tank. After breeding, transfer them back to the main tank to leave their younglings to grow without being feasted on or interfered with.

How to Care for Mollies in a Group

White molly fish

You must provide the best conditions possible for your mollies in a group to thrive. Here are the best ways to care for your mollies in a group:

Ensure Enough Space

Tank size and setup matter for a comfortable and active lifestyle for your molly fish. 

Generally, adhere to the molly fish tank size chart indicated earlier in this article. In a nutshell, there’re small and large tanks for your fish, depending on the size of the fish you want to keep.

That said, a rule of thumb is to keep fewer fish in ample space for efficient growth. A 10-gallon (37.850-liter) tank should have two or three mollies, and a 20-gallon (75.71-liter) tank four to six fish.

Ensure They Live in a Pair or Group

Social and active, mollies should be kept together in a group. They are a schooling fish by nature – so keeping them single can be detrimental to their health and well-being. A pair is the least unit you should keep your mollies in.

Otherwise, keep three, four, or five, depending on the tank size. 

Staying together is beneficial to your molly fish. It improves the fish’s overall welfare, including stress reduction and the promotion of healthy natural behavior.

That said, expect a raft of issues including stress, competition for light and food, nipping, and disease if you keep many mollies in one tank beyond its capacity. Therefore, be careful when planning for the correct number in a tank.

Provide a Healthy Diet

Diet and feeding are key pointers to a happy and healthy-looking molly fish. 

You should provide a well-balanced, high-quality diet throughout your fish’s lifespan. The diet supplied should be enough for all the mollies in the aquarium to avoid competition for food.

Some of the best molly food for a healthy fish include:

  • Frozen bloodworms
  • Brine shrimps
  • Daphnia
  • Flaked pellets

You can also prepare veggies like blanched zucchini and squash and give your mollies to supplement the algae that’s readily available in the tank.

Use an Efficient Filtration System on the Aquarium

The key to successful molly fish care is clean, fresh, and well-oxygenated water. 

If you have more than one fish in the tank, provide a high-quality filter to purify the water and keep it clean always.

Moreover, don’t forget to maintain the filters to ensure they keep working without failure. As you can already see, filters will determine your fish’s health.

In a nutshell, the filtration system keeps in check the quality of the aquarium water.

Check on the Water Chemistry

It’s vital to check on the aquarium’s water chemistry. 

For instance, check and adjust the following parameters accordingly to ensure your fish is healthy and happy:

  • A temperature range between 75 and 82℉ (23.89 and 27.78℃)
  • A pH of around 7.0 to 8.5
  • Slightly hard water is preferred for molly fish

Final Thoughts

When it comes to the question, “How many mollies should be kept together?” there is an array of factors to consider. 

From tank size and compatibility to breed and growth stage, each factor plays is essential in determining the optimal number of mollies for a thriving aquarium community. Therefore, assess these factors carefully to ensure the well-being and happiness of your mollies. 

Finally, before adding more mollies to your tank, ask yourself, “Are you creating a harmonious aquatic world or upsetting the delicate balance? The choice is yours, but remember, the magic lies in finding the perfect equilibrium for these graceful creatures to flourish.

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