How Many Mollies in a 29 Gallon Tank?

Raising mollies in a 29-gallon tank can provide them with the right conditions to grow and thrive. A 29-gallon tank is spacious enough to house up to nine mollies without hindering their movement or preventing them from being their active selves.

In this post, we cover all you need to know about raising mollies in a 29-gallon tank, from why a 29-gallon tank is an ideal size for raising mollies to why you shouldn’t overcrowd your tank.

How Many Mollies in a 29 Gallon Tank?

Based on the general “one inch of fish per gallon” rule, you could house seven to nine mollies in a 29-gallon tank depending on their gender and breed.

This is taking into consideration that the average molly size is 4 inches for females and 3 inches for males.

Generally speaking, a 29-gallon fish tank is ideal for housing and raising mollies, as it’s spacious enough to accommodate their energetic and socializing needs.

Mollies are very active fish that don’t like to feel confined and like to swim around freely. Raising mollies in a spacious 29-gallon tank ensures that they have the room needed to swim around as they would in nature. This prevents mollies from being stressed due to a lack of space.

A 29-gallon tank also allows the fish to school and practice the habits they would in nature. This ensures raising happy mollies that will flourish and lead long, healthy lives.

Molly Fish

Is This Rule Completely Accurate?

While this rule is a great tool to give you a general idea of the number of fish you can keep in a tank, it’s not completely accurate and should be taken with a pinch of salt.

There are two important factors that this rule fails to take into consideration: variation in fish size and the sizes of decorative items.

This rule uses the average fish length to calculate the suitable tank size, but it doesn’t consider the fact that some fish could grow a little longer or shorter than average. It also doesn’t account for the fish’s height.

Another point that this rule misses is the size of the decorative items. Fish tanks usually contain plants and rocks that take space and need to be taken into consideration when calculating the suitable tank size for your fish.

How Many Male and Female Mollies in a 29 Gallon Tank?

As a general rule, for every male, there should be two female mollies in a tank. This is crucial in ensuring peace in your tank, as male mollies become aggressive and very persistent when they need to mate.

Having two females for each male ensures that this aggression and extra attention is spread out between the two females so they don’t get overwhelmed.

Note that an overwhelmed female molly might suffer from stress, which could become fatal.

A 29-gallon tank is big enough to house many mollies comfortably. It’s ideal for two males and five female mollies while leaving enough space for the mollies to swim around calmly.

Pro Tip: Keeping some bushy plants in your 29-gallon tank will provide a space for females to hide from male mollies when they need to escape their attention. The plants will also make for a pleasant decorative item for your tank.

Can You Raise Different Types of Molly Fish Together?

There are several different types of molly fish, like black, golden sailfin, and dalmatian mollies, and each has its own looks and temperaments.

Is it possible to raise different types of mollies together, though? Or will there be conflict in the tank?

Different types of molly fish can co-exist together happily and can even mate, but you have to know what you’re doing before putting different types of mollies together.

Sailfin Molly

Why You Shouldn’t Overcrowd Your 29 Gallon Tank

While a 29-gallon is a big tank that can comfortably house many mollies, it’s still important not to overcrowd your tank.

An overcrowded tank can cause many problems; some of which can prove fatal for your mollies, including:

Stress Leading to Disease

An overcrowded tank will cause mollies to suffer from stress. This stress will eventually cause several health problems for your mollies.

Just as it does for humans, stress will lower the mollies’ immunity system, making them more susceptible to many bacterial and fungal fish diseases.

Hindering of Natural Growth

When mollies live in overpopulated aquariums, their natural growth process might get affected.

Overcrowded tanks, as well as poor water conditions, are the main causes of stress for mollies, which negatively affects their appetite and eating patterns. This ends up hindering their natural development, causing them to grow at slower rates than usual.

Ammonia Buildup

Ammonia is one of the most common causes of mollies’ mortality. An overcrowded tank causes ammonia spikes, which leads to poisoning of the tank water.

When ammonia toxicity occurs in the tank water, mollies will start to exhibit unusual behavior like loss of appetite, gasping at the tank’s top, or laying at the tank’s bottom. You may also notice red and inflamed gills on the fish.

Ammonia poisoning will cause organ failure or brain damage to mollies, which leads to death.

Nitrites Buildup

Nitrite levels usually rise in correspondence with the ammonia levels rising in tanks. Although less toxic than ammonia, nitrite can still be fatal for mollies if its level rises in your tank.

Nitrite poisoning is also called “brown blood disease” as the infected fish’s gills will usually turn brown. Mollies that suffer from nitrite poisoning will appear to be gasping for air by either moving to the top of the tank or by the rapid movement of their gills until they eventually suffocate.


One of the most common problems of overpopulated tanks is fish aggression. Mollies, who are peaceful fish by nature, will start showing signs of aggression and attacking one another when they don’t find enough space to swim around freely in their tank.

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 20-Gallon Tank?

Bottom Line

A 29-gallon could be an ideal tank size to raise your mollies. It’s big enough to house up to nine mollies while providing them with enough space to swim around and socialize, just like they do in nature.

While the 29-gallon is a big tank, you still shouldn’t overpopulate it to ensure that your mollies thrive and live a healthy life.