How Many Mollies in a 20 Gallon Tank? A Guide for New Fish Owners

Mollies are some of the most popular fish to raise because of their energetic and friendly nature. A 20-gallon tank is an ideal size to raise five or six mollies while allowing them to flourish and live harmoniously. 

In this article, we’ll explain why a 20-gallon tank is a great option for raising mollies and why you shouldn’t overcrowd your tank.

How Many Mollies in a 20 Gallon Tank?

There’s a general rule used to determine the number of fish that can be kept in a tank, which is the “one inch of fish for every gallon” rule. While this rule is useful in having a basic idea of how many fish to keep in your tank, it is not perfectly accurate.

It’s worth pointing out that this rule doesn’t account for rocks, plants, and other decorative items put in the tank. In addition, it does not consider the fish’s height, as well as the fact that some fish could grow to be longer or shorter than the known average size.

It’s a useful rule to use, but it’s not perfect!

The average size for a molly fish is 3 inches for males and 4 inches for females. Using the “one inch of fish per gallon” rule, you can house an average of five or six mollies in a 20-gallon tank.

Black and Yellow Molly Fish

How Many Female and Male Mollies in a 20 Gallon Tank?

It’s ideal to have two females for every male molly.

Male mollies tend to be aggressive and pushy when mating, so it’s advised to have two females keep them company. This will help spread out the male’s attention, giving the females a chance to rest and protecting them from getting harassed, which leads to stress.

Since a 20-gallon tank can accommodate up to six mollies, it’s ideal to have two males and four females in your tank.

If you’re raising larger molly breeds in your 20-gallon tank, like sailfin mollies, it’s better to house a maximum of four or five mollies, with the number of females exceeding that of the males.

It’s also recommended to keep some bushy floating plants in your 20-gallon tank. This provides a necessary hiding place for female mollies that can be used to escape the males when they’re being too pushy or aggressive. Plants also make your tank look more aesthetically pleasing.

Are 20 Gallon Tanks Suitable in Size for Raising Mollies?

Molly fish are energetic and friendly fish that require enough room to swim around and other fish to school for them to flourish. Therefore, it’s recommended to house mollies in big tanks, making 20-gallon tanks the ideal size to start with.

A 20-gallon tank has enough space to allow for housing four mollies, which is the minimum number of fish required for them to practice their unique schooling habits similar to what they do in nature.

A 20-gallon tank also provides enough swimming room for the mollies, ensuring that they’re their active selves and that they’re not at risk of stress from lack of space.

Reasons Not to Overcrowd a 20 Gallon Tank

While a 20-gallon tank is a suitable size for housing mollies, overcrowding the tank can still cause many issues, including:

Behavioral Issues

An overcrowded tank is sure to negatively affect the mollies’ behavior. While mollies are active by nature, living in an overcrowded tank could cause them to become lazy and dull. The mollies will spend most of the time hiding in plants instead of swimming and socializing with each other.

An overcrowded tank may also cause mollies to become aggressive and fight one another. Aggressive mollies are a danger to themselves as well as to their tank mates, as fighting might lead to their injury or they might even kill each other.

A closeup of a black and gold molly swimming in an aquarium under the lights

Stress Issues

When a tank is overcrowded, mollies will start to suffer from stress and anxiety. When mollies are under stress, they will suffer from many health issues. Stress will lower the mollies’ immunity, causing them to become at risk of many diseases.

As the mollies’ immunity system is compromised, they become vulnerable to bacterial and fungal infections, some of which are fatal.

Stress will also cause the mollies to excessively secret hormones from their adrenal glands, which causes an increase in blood sugar levels. This could also end up being fatal for mollies.

Growth Issues

While many people believe that the size of the tank negatively affects the mollies’ growth, it’s mostly related to overcrowding and not the tank size, per se.

Overcrowding a tank will usually cause mollies to become stressed, while also affecting the water’s cleanliness and overall condition. These reasons could end up hindering the mollies’ natural growth process.

The stress caused by overcrowding could also lead to the mollies’ loss of appetite and change in eating habits, which could also halt the fish’s growth.

Oxygen Level Issues

An overcrowded tank may not provide enough oxygen to the inhabiting mollies. Having low oxygen levels in the tank is a very serious problem with a high risk of fish mortality.

As the oxygen is depleted from the tank, mollies will start to move less and become lazier and more static, unlike their natural instinct of being active all the time. Mollies will also eat less, causing them to be at risk of starving.

With low oxygen levels, mollies will start showing signs of rapid gill movements, as well as surfacing to try and gasp for oxygen. These are signs of oxygen-deprived mollies that are at the risk of imminent demise.

Have Another Tank Size You’re Looking for?

How Many Mollies in a 5-Gallon Tank?

How Many Mollies in a 10-Gallon Tank?

How Many Mollies in a 29-Gallon Tank?

Bottom Line

A 20-gallon tank is a great tank size to raise mollies. It’s spacious for housing five or six mollies, allowing the fish to school and to exhibit the unique behavior that they do in nature. 20-gallon tanks are also big enough to allow mollies enough space to swim around freely.

However, it’s crucial not to overcrowd the tank, as it could lead to many health and behavioral problems for the mollies. It’s also important to keep two female mollies for every male to prevent the risk of aggression problems.