How Many Mollies In a 5 Gallon Tank? A Quick Guide

Mollies are among the most common pet fish because of their beautiful colors and calm temperament and are considered beginner-friendly fish to raise. Raising one molly fish in a small 5-gallon tank is possible. Though it’s never a great idea since 5 gallons is a bit too small for a molly.

Mollies are very energetic fish and require a spacious tank to swim around freely. They also lead their best lives when being part of a school. In this article, we’ll explain the best conditions to raise a healthy Molly fish.

How Many Mollies in a 5 Gallon Tank?

Assuming that the average size of a molly fish is 4 inches, and using the rule of “one inch of fish per gallon,” it’s advised to keep only one Molly in a 5-gallon tank.

Mollies, however, are very friendly fish that flourish when having company. So while on paper, mollies can survive alone in a 5-gallon tank, it’s not recommended to get a molly fish if this is your only tank size option.

Yellow mollies in an aquarium isolated on a black background

General Rule For Number of Mollies Per Tank Size

There’s a rule of thumb: “one inch of fish per gallon.” This rule determines the number of fish you can keep in your tank according to the size. However, this rule isn’t perfectly accurate and can only be used to estimate and give a basic idea of the tank size needed.

It is also worth noting that the formula above does not account for the size of the decorative objects in the tank, like plants and rocks. It also does not account for mollies that could grow to different sizes and not necessarily the common average size.

What Is a Molly’s Average Size?

There are about 43 species of mollies, but only three are available for the aquarium trade. While mollies vary in colors and shapes, they almost all share the same size. For example, the female Molly can grow up to four inches in size, while the male Molly only grows to three inches.

Can A Molly Fish Live Alone?

Mollies are sociable fish that cannot flourish alone. While a single molly fish in a 5-gallon tank may live for a month or two, it won’t survive for a long time.

While we never think of fish as creatures with feelings, that is very untrue. A single Molly will experience loneliness, which could cause many health and behavioral issues and significantly shortens its life.

Mollies prefer to live in a school of at least six fish.

Can Another Breed of Small Fish Live With A Molly In a 5 Gallon Tank?

While many fish breeds can make good tank mates for Mollies, like Gouramis, Plecos, Angelfish, and Corydoras, it’s recommended to put them together only in case of a bigger tank.

Mollies are very friendly fish and could co-exist with many breeds, but they require enough space so they can live calmly and don’t display behavior problems.

A 5-gallon tank is barely enough for one Molly to live and lead a healthy life. Having another fish with a Molly in a 5-gallon tank is bound to cause many issues and is definitely a big no.

It’s recommended for mollies to live in a bigger tank and with the company of other fish for them to thrive.

What Happens If You Overcrowd a 5 Gallon Tank?

An overcrowded Molly tank can cause just as many problems as a Molly living alone.


An overcrowded tank of mollies is sure to put the fish under a lot of stress and anxiety. When Mollies become stressed, they usually swim around more actively. Having an overcrowded tank doesn’t help with this situation. Instead, it causes the Mollies even more stress.

When Mollies suffer from stress, they become at risk of many health problems and may end up having a shorter lifespan.

Behavioral Issues

An overcrowded tank causes Mollies to exhibit unusual behavior. While Mollies are good-natured and calm fish by nature, when living in an overcrowded tank, they will become aggressive towards one another.

Aggressive Mollies are not only dangerous to their tank mates but can also end up harming themselves.

Molly fish on a black background

Hindered Growth

While being in a small tank doesn’t necessarily affect a Molly’s growth, its low activity and living conditions can directly affect its growth.

Overcrowding a 5-gallon tank will restrict the Molly’s movement, which in turn will hinder its natural body’s development process and considerably slow down the growth rate.

Poor Maintenance

An overcrowded small tank will get dirty quicker than usual. This requires extra effort in maintaining the tank, which, if not met by the fish owner, could become harmful to the Mollies.

An overcrowded and polluted tank means more harmful bacteria and microbes, which could cause illness or even become fatal for the fish.

What Is The Ideal Tank Size And Conditions For Mollies?

Mollies are energetic and friendly fish, meaning they need enough room to swim around and other fish to keep them company. Providing these conditions is essential for raising healthy Mollies.

Raising four Mollies in a 20-gallon tank is a good place to start. This ensures that the tank isn’t overcrowded and that the fish have enough room to swim around calmly. A spacious tank prevents the fish from getting stressed or aggressive with each other.

It is also recommended to have three females and one male Molly. When a male Molly has only one female to mate with, they can become too determined, which stresses out the female. Having two females for each male is the recommended ratio to go by.

Have Another Tank Size You’re Looking for?

How Many Mollies in a 10-Gallon Tank?

How Many Mollies in a 20-Gallon Tank?

How Many Mollies in a 29-Gallon Tank?

Bottom Line

To sum things up, it’s never a great idea to raise one Molly alone in a 5-gallon tank. This small tank doesn’t have enough space to host two Mollies, and while a single Molly can exist, they become lonely and lead unhealthy brief lives.

It’s recommended to use at least a 10-gallon tank, which has enough space for three Mollies, allowing them to co-exist peacefully and to have enough room to swim around calmly.