Skip to content

Number of Mollies vs. Tank Gallon Size [Complete GUIDE]

Being an aquarist, you should first have the ideal tank size sufficient for the movement and schooling of Mollies.

The easily bred and most popular fish for aquariums is the Mollies. Luckily, Mollies don’t acquire large tanks for a living but need sufficient space.

Below is some interesting discussion on how many Mollies can be placed in a 10-gallon tank.

A 10-gallon tank is usually not recommended for all kinds of Mollies because of their different sizes (3-6 inches) and super playful nature. One inch per gallon is also controversial in the case of Mollies, but still, we don’t reject a 10-gallon tank. For a Mollies of 3-inch size (Blackfin Mollies), you can add 3 of them in a 10gallon tank. Try to avoid a 10-gallon tank for the Mollies having a size of 6inches.

The answer given above is not enough for such a broad topic. Continue the reading, and you will find all kinds of Molliwa with their size and number in a 10-gallon tank, the male-to-female ratio in a single tank, and much more.


Following is the table of Mollies with different kids, sizes, and numbers in a 10-gallon tank.

Type of MolliesMaximum SizeQuantity in 10-Gallon tank
Black Molly3 inches3
Sailfin Molly5 inches2 (Big tanks are recommended)
Dalmatian Molly4.75 inches2 (Big tanks are recommended)
Lyretail Molly3 inches3
Doubloon Mollies5 inches2 (Big tanks are recommended)
Types of Mollies Vs Maximum size and Quantity in a 10-Gallon Tank

Male to female ratio for a 10-gallon tank

In breeding season mostly male Mollies chase and attack female Mollies which creates a stressful enviornment inside the tank. To tackle this issue I recommend the number of female must be greater then that of male (I recommend two females for a single male).

Bushy floating plants offer molly fry a place to hide. Hornwort is suggested. It will house and grow microorganisms that your fry will nibble on. It also improve water quality.

Avoid small tanks with large amount

Molly is a very interesting and active kind of fish for your aquarium. They need enough space to grow. Small tanks with large amount of fishes will limit their movement, compitition for food and space will generate, maintainance of water quality will be challanging and the plants and hiding place will make the tank more conjusted. I recommend to follow the table given above for adding Molly in a tank and having 15% fewer Mollies in overcrowded tanks is favorable for avoiding conflicts and stress conditions.

Taking Care of the Mollies in a 10-gallon tank

A 10-gallon tank is enough for some kinds of Mollies. So to keep your Molly alive and happy, you need to take good Care of the tank/water parameters.

1. Plantation

Make sure you have enough plants in your tank to absorb the carbon dioxide, nitrate, and other substances that are toxic waste from the fish excretion.

So plantation in the ten-gallon tank is essential.

Plants in Aquarium

2. Water Quality Check

Change the water frequently to eliminate all the toxins. If you have a filter, change the water weekly.

If you do not have a water filter, about 40% of the tank water should be changed daily.

3. Feeding Habit

When feeding your Molly, try not to put too much food as it can pollute the water. Stop feeding if they stop eating.

4. Water filter installation

You need a filter that will eliminate the ammonia by changing it to nitrite and then nitrite into a less toxic form called nitrate.

(Nitrates are removed when you do the water changes in the tank). This process is called the Nitrogen Cycle.

5. PH Level

Also, make sure to maintain a healthy PH level in the tank. Here is how you can naturally control the PH in your water tank.

Pros and Cons of keeping Molly in a small VS large size Gallon tank

1. Tight space

If you live in a small house or have little space, a ten-gallon tank is perfect for carrying with your Molly hobby. For large homes, a large-sized tank minimum of twenty-gallon will be preferred.

2. Indoor Aesthetics

Being a colorful fish, Molly will improve your home aesthetics, and all this is set in a low space occupied.

3. Low maintenance

The significant edge of keeping Mollies in a ten-gallon tank is the ease of maintenance of this size. Low maintenance will lead to a pleasant environment for your Mollies to dwell in.

4. Aggression

A confined space cause behavioral issues like aggression in the Mollies. Aggression towards its tank mates is a common problem that attacks other fish. This unfriendly environment inside the tank can lead to Molly death. There are no such issues in large-sized tanks.

5. Waste toxin accumulation

Due to less space, more toxins fromMollies waste accumulate inside the small sized tank. These toxins disturb the water chemistry.

6. Poor growth

The growth of Mollies organs is poor due to the confinement of space. They can’t swim freely because poor growth leads to organ failure and premature death.

How many Mollies in a 20, 30 and 50 gallon tank?

If you are an experienced aquarist, you can manage a big-size tank smartly. Otherwise, beginners are requested to learn about the mollies first and apply their knowledge in a small tank of ten gallons. After a bit of experience then work on a big-size tank

Type of MolliesMaximum SizeQuantity in 20, 30, 50 Gallon tank
Black Molly3 inches6,8,15
Sailfin Molly5 inches4,6,10
Dalmatian Molly4.75 inches4,6,10
Lyretail Molly3 inches6,8,15
Doubloon Mollies5 inches4,6,10

Mollies in a 5-Gallon Tank

According to my experience, I don’t recommend a 5-Gallon tank for Mollies. Those who used 5-Gallon tanks for mollies also had a bad experience.

Tank Mates of Mollies

Cichlids, Discus, Severums, Platies, Neon Tetras, and Angelfish are the frank tank mates of Mollies.


Mollies are a lovely collection for your tank, and you can add Mollies in a ten-gallon tank, but caring for your aquarium will boost your interest and increase the life of your Molly. They are a beautiful collection for your tank. Try to avoid big-sized Molly in a ten-gallon tank.