How Many Molly Fish In a 5, 10, 20, And 30-Gallon Tank?

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One of the most common pet fish, Molly fish, is also a difficult species among the common species if you don’t know its requirement. As small and delicate as it looks, it requires great care too. Today, we will be talking through how many molly fish you can put in your tank, and how you can provide them with a good environment to survive.

People usually add decorative items like aquatic plants, gravel, driftwood, and many more, to make their tank look pretty. However, many of us don’t realize that these accessories take up the aquarium space, decreasing the water availability for its residence. I am not talking or giving you any sort of advice to not to keep decorative items but to calculate the area it takes while calculating the space that the fish needs. One inch of fish requires one gallon of water to survive. So, for an adult molly fish, that’s 3 inches in length requires 3-Gallon of water. Let’s suppose One-Gallon of your tank volume is covered by the decorations leaving you 9 gallons behind. So, that’s (10-1)/3=3; you can add three molly fish in a 10-gallon tank.

All about Molly fish: A short description

A community fish from the genus Poecilia, a molly fish, is the one that most of us have seen at least once or have been petting it at least once by the keepers. This genus has about 39 species of mollies, but only three of these species are available in the aquarium trade. Varying in shapes and colors, size ranging from 2-3 inches, peaceful in nature, mollies have an attractive appearance with a lifespan of up to 5 years.

Among the species, the Sailfin Mollie is undoubtedly the most beautiful and exotic fish. Originating from Mexico, Central America, and in the northern parts of South America, these small-sized tropical fishes are hybridized for a more colorful and better specimen. They can be differentiated from other fishes through their flattened body, which is narrow towards the mouth and tall in the middle part. It isn’t too difficult to tell the males from the females, which makes their mating easy. The prime difference is that the females have an anal fin that spreads into a fan, whereas in the males, the anal fin is pointier. Another differentiating characteristic is the body size; the females can grow up to 4.5 inches, but the males only up to 3 inches.

The most common mollies for home aquariums include Black Molly, Sailfin Molly, and Lyretail Molly.

Nature of the Mollies

Mollies are very peaceful, but they can be aggressive when they are put in together with the aggressive-natured fishes of other species. They tend to be protective about themselves at other times, so it is safe to call them a community fish, for they do not harm their tank makes.

Like most of the species, Mollies also live in groups, known as schools. These schools of mollies search the food near the water surface. The females are less aggressive towards other fishes than the males.

Habitat of the Mollies

Mollies are typically freshwater inhabitants but are also compatible with brackish coastal sea waters, saltwater, and swamps. Over the years, they have adapted to several water conditions, so they can also be found in rivers, small pools, wetlands, streams, and coastal regions.

In a home aquarium, the mollies are suggested to be put in a tank with water hardness 15-30 dGH with the sandy substrate at the bottom and live plants to create a natural environment for them.

Water Temperature

To fishes like Mollies living in natural water sources, you need to be very careful regarding its living conditions inside the tank. You should always keep in mind that the water they naturally inhabit is slow-moving, warm, and slightly alkaline. So, their artificial inhabitation inside the tank should be a freshwater of temperature anywhere between 70°F-82°F (21.11°C-27.78°C).

Tank Size of Mollies

As easy as Mollies seem to be kept, you need to make sure of certain tank requirements to keep your small babies healthy and happy. These tiny creatures have a light diet. They feed on the bits of food, flake food, live or frozen food like blood-worms, brine shrimp, and daphnia.

You should be very cautious about the number of Mollies you keep in the given tank size and not overcrowd the tank. I repeat, DO NOT overcrowd the Mollies in your tank. It can lead to fights between the breed themselves.

One inch of a molly requires a gallon of water. Depending upon the size of the species, you need to choose your tank size. For four of the mollies, 10-gallon water will work. But if you have Sailfin mollies, then you would require about 30-gallon of water for four of them [may vary according to the size]. Tall aquariums are recommended for them.

How many molly fish in a 5-gallon aquarium?

Since overcrowding is specifically suggested to avoid, and with creating the landscape using substrates, live plants, rocks, and caves, space is occupied by the decorative also needs to be considered. So, it would be wise of you to keep only one molly in a 5-gallon aquarium.

But from my personal experience and knowledge, I suggest you rather not keep your molly alone.

How many molly fish in a 10-gallon aquarium?

With the growing tank size, the number of mollies you can keep also increases. Sounds exciting? It is, but it doesn’t give you the liberty to add many as you want. The numbers have to be limited. In a 10-gallon aquarium, you can add a maximum of 3 Mollies. Say one molly is 3 inches, so 3-gallon of water for that fish only. So, if they have a standard size of 3 inches, then 3×3= 9-gallon of water for 3 Mollies and remaining 1-gallon for the accessories that take up the tank space. According to the size, we can see that the number of Mollies that can be put in the 10-gallon can vary. The purpose of allocating the water as per the size of the fish is because they need a huge space to swim.

How many molly fish in a 20-gallon aquarium?

In a 20-gallon aquarium, 20/3= 6.67. Rounding off, we can see that up to 6 Mollies of 3 inches can be added. If the molly fish is of 4 inches, then you can add 20/4= 5 Mollies in a 20-Gallon tank. But to add five fishes would be compact, so reducing the number to 4 Mollies is advised. If your Mollies are of 2 inches, then you can add up to 9 Mollies. But the size of each molly doesn’t have to necessarily be the same.

How many molly fish in a 30-gallon aquarium?

Calculating similarly, you can add up to 30/3=10 Mollies in a 30-gallon tank. But if you are choosing a Sailfin Molly, then you will require a 30-gallon aquarium, as they need a lot of space to swim.

I believe that you have by now noticed how I have been suggesting you keep the Mollies in even numbers. It is better to keep them in pairs, preferably keeping the number of both gendered Mollies the same. Or follow the rule of thumb, “two females per male” to reduce the chances of bullying.

How many Molly Fish your tank can accommodate?

The number of Mollies that your tank can accommodate depends on your tank size. So, for your convenience, you can calculate the number of Mollies you can keep in your tank by yourself.

The trick to find this out is “Size of the tank ÷ Max Length Of a Molly Fish = Number of Mollies”. 

In this, we first choose the size of the tank. Then, we take the number of Molly fish we want to add or suppose a certain number of them. Take either two females per male or an equal number of both genders. Then, measure the length of each of them; it is easier to round off the decimals. Add the length of all the fishes. Then divide the size of the tank by the sum of the length. Finally, subtract the result from the size of the tank. This gives you the total length of Mollies that you can keep in your tank.

For example

If the size of your tank is 50-gallon and the maximum size of the molly fish is 2 inch (balloon molly), now we calculate the number of mollies:

Number of Mollies= 50/2= 20

This means that the total inches of Mollies that can be added in a 20-gallon tank are up to 19 inches (round off). Following the rule of thumb, we can add either three female Mollies and two male Mollies. This will give us a total of 18 inches of Mollies (3*4+2*3).

Your Tank size (LxHxW) Approx Volume* Remarks
24 x 12 x 12″ (61 x 30 x 30cm) 10 gal (45 L) up to 4 Mollies at most
36 x 15 x 12″ (91 x 38 x 30cm) 20 gal (89 L)  6-8 Mollies
36 x 18 x 15″ (91 x 46 x 38cm) 30 gal (137 L) up to 14 Mollies

Maintaining your Molly tank

As essential it is to keep the number of Mollies limited, we can not forget other important factors that we need to focus on to keep them in good shape and health. In slow-paced but warm water sources like streams, pools, and coastal regions, these little creatures are found swimming freely. The constant water temperature between 70°F and 82°F should be maintained. It can range anywhere between these temperatures, keeping in mind the temperature requirements of other fishes living in the tank. The temperature should be kept at a point where all the inhabiting fishes are comfortable to live in. Whether you keep them in brackish fish tanks or freshwater aquariums, the pH level of water must be in the range of 7.5-8.5, whereas the hardness of the water ought to be around 15-30 dGH.

Maintaining your Mollies’ diet

As tiny as they are, you might have already guessed the diet of these little creatures. Known to eat more than they need to, you need to be very careful not to overfeed your Mollies. They need to be fed 2-3 times a day. These omnivores can be fed either live food or artificial food, even vegetables. In their natural habitat, they feed on plants, algae, and small invertebrates. Nonetheless, you can tear your kitchen vegetation like lettuce, spinach, and zucchini into small pieces and dropped in the aquarium. You have the flexibility to combine them to have your DIY fish food.

As for live and frozen food, they prefer brine shrimps, bloodworms and daphnia, whereas, for artificial food, the easy option would be pellets and flakes. For healthy digestion of the Mollies, I would personally suggest you to only feed the amount of food that can be eaten in about 3 minutes.

Water Filtration for the tank

We don’t for sure about the need to mix salt in the water yet. Regardless, the regular change of water is a must-to-do task. Not only for Mollies, but the water in the tank containing any species should also be changed frequently. Mollies are prone to various health conditions due to sudden alteration in water quality. You, as an owner, have to be very cautious during this process.

A filtration system is always recommended and required for any aquarium.

Mollies produce a lot of waste, hence contaminating the water, the plants and affecting other fishes too. Not only Mollies, all fishes do this, but it’s equally important to maintain water hygiene. It’s like maintaining pollution-less air for humans and other terrestrial beings. The effect of polluted air might not be seen immediately, but it goes on degrading health. But fishes are more delicate than other animals and need extra proper care. So, adding a filtration system is a smart move to remove debris like excess food particles, fish excretions, bacteria, and harmful chemicals.

Even though the filtration system sounds a necessity, it can be an option for the Mollies, if you have good hands with experience and have been a keeper for years. It might be easy for you to remove the waste with perfection, but for a better cleaning process, an aquarium filter is recommended.

Decorations for your Mollies

As I’ve been mentioning repeatedly regarding the accessories and decorations that you can add in your aquarium. The extra additions assist in creating a better and more natural look-alike living environment for your fishes. You can add sandy substrate, gravels, stones, glass pebbles just to add some color. Not only that, but live or artificial aquatic plants, rocks, corals, driftwoods, and many more can also be added. These add essence and make your tank look more attractive, and the Mollies can use the plants and rocks as shades.

Does the breeding of molly matter when choosing tank size?

Yes! While choosing the tank size, you need to decide if you want to breed your Mollies or not. They can produce 10-140 juveniles. If you need 30-gallon of the aquarium for 14 Mollies, then we cannot even imagine the tank we would require for 100 of them. Unless it is for a commercial purpose, be careful when you choose the tank size.

Keep your eyes on them if you somehow plan to breed your Mollies, or if they mate on their own.


From my experience, Mollies are a lovely species of pets. A little bit of attention towards the number of Mollies you pet in your tank can give your eyes the pleasure of seeing these beautiful babies grow.

Overcrowding is a big NO. I hope that this article has been a guide to you for adding the Mollies in your tank. Irrespective of the tank size you own, a bigger tank doesn’t mean that you add the Mollies in them thoughtlessly. It can be risky not only for them but for your other fishes as well. Mollies need space to swim freely and remain healthy. I have loved being a keeper of the Mollies due to the ease in feeding them and nurturing them. They are small and beautiful. Moreover, their peaceful nature makes it comfortable to be added with other tank mates as well.

If you want to add Mollies to your collection and want to learn more about them, you can read our article on Mollies in the given link.

Happy fish keeping!


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