How Big Do Molly Fish Get & How Many Can You Keep?

Last Updated on 2023-09-20

Mollies come in a variety of colors, including black, white, silver, red, and orange, among others. Molly fish are a popular choice for people who have never kept fish before because of their ability to thrive in a diverse range of environments. Mollies are laid-back fish that get along well with others, making them an excellent choice for a community aquarium because of this trait.

If you are thinking about getting a Molly fish for your aquarium, there are a few things you need to be aware of before you make the purchase. In order for you to be able to make an informed purchase decision, we are going to provide you with the fundamental knowledge of fish that you will need. When they reach their full maturity, mollies grow to be about what size? Continue reading to find out more information!

How Big Do Molly Fish Get?

Be aware that there is not just one right answer to this question. There are multiple. This is due to the fact that Molly fish can be found in a wide range of different shapes, sizes, colors, and other characteristics. On the other hand, we are able to build a full database of Molly fish species and provide a detailed description of their maximum sizes. As a result, you might be able to make a more informed choice about what to purchase, or you might even purchase more than one type.

Dalmatian Molly

This particular specimen of black and white molly fish is a hybrid that was produced through the process of selective breeding but did not occur naturally. Its unusual body pattern resembles the coat of a dalmatian dog, with black spots and speckles set against a white background, like in the case of the dalmatian dog. When completely mature, this species of fish can grow to a maximum length of approximately 11.75 centimeters (4.75 inches).

Black Molly

There is no more well-known and widely kept molly than this one. Black molly fish are the ideal fish for first-time fish keepers. It has lower requirements in terms of tank size and maintenance, is simpler to breed, and has lower breeding success rates.

The fish is quite long and narrow at the base, with a slight flattening toward the tail. It has big eyes and a big, upward-facing mouth. Within 6 months, males can reach a length of 3 inches (6-7.5 cm), while females can reach a size of 4 inches (10 cm). Only in the second year of a fish’s existence can its fins reach their full beauty and size.

Black Sailfin Molly

A dark variant of the common Sailfin Molly, the Black Sailfin stands out. One of the most popular breeds because of its attractive appearance. This kind of livebearer needs specific environmental conditions in order to flourish. The best home for them is a densely planted aquarium or a thick mat of plants. It reaches a maximum length of around 6.5 inches, is always black, and, as its name suggests, sports a unique dorsal fin shaped like a sail.

Black Lyretail Molly

The Black Lyretail Molly is the result of a hybridization between the Sailfin Molly, also known as Poecilia latipinna, and another species. The Black Lyretail Mollies caudal fin is shaped like a lyre, giving the animal its common name. Mollies in an aquarium may develop a tolerance for varying salt levels over time. The Black Lyretail is a mysterious species that may grow to a maximum size of approximately 5 inches in length.

Little Molly fish, Poecilia latipinna in fish tank or aquarium, underwater life concept.

Gold Dust Molly

Gold Dust Molly, who is black and gold in color, possesses a lively and happy demeanor. Its eye-catching color pattern and the sociable behavior of its inhabitants make it a remarkable addition to any aquarium. Gold Dust Mollies were quite active in the water and would frequently chase after one another. This fish, true to its namesake, is golden yellow in color and resembles a goldfish in appearance. It has a height/length range that could be anywhere between 5 inches.

Balloon Belly Molly

It has a huge stomach and a shorter body. The skin tone could be orange, white, red, or black. Due to its diminished and compressed interior, this molly is the weakest. The curved spine of the fish makes them pickier about their tank environment, and as a result, their average lifespan is only 1.5 to 2 years. Despite that, these colorful fishes are a great way to add some variety to your tank and are fairly popular. It has a maximum potential length of about 3 inches.

Marble Lyretail Molly

Mollies like this one are attractive, and they don’t bother the other fish in aquariums. Because of its affinity for hard, alkaline water, the Marble Lyretail Molly may not perform well in aquariums that have water that is acidic or soft. This serene Molly fish has a beautiful marbled design that consists of blue, white, and black colors. It is possible for it to grow to a maximum length of approximately 5 inches under the right conditions.

Gold Doubloon Molly

The fact that they are so well-liked by enthusiasts has led to the creation of a great deal of variety in terms of colors and the structure of their fins. This particular Molly has a remarkable color gradient that goes from pitch-black at its tail end to a brilliant golden yellow at its forehead. The color gradient can be seen in the image to the right. The Gold Doubloon Molly may grow to a length of up to 5 inches if given the opportunity.

Harlequin Sailfin Molly

Harlequin Sailfin Mollies are characterized by having a combination of golden, white, and black patches spread across their body. These fish are not only striking to look at as a result of the vivid colors they possess but also have a laid-back temperament that makes them an excellent choice for living in an aquarium with other species.

They grow to a somewhat greater size and consume a wider variety of food than ordinary molly fish, which likely contributes to this adaptation. This fish can grow up to a maximum length of approximately 15 centimeters (6 inches).

Platinum Lyretail Molly

The Platinum Lyretail has scales that are both silver and iridescent, which causes it to shimmer when exposed to light. It glides across the water with grace, thanks to its forked tail. However stunning it may be, keeping it in good condition calls for very little work or expense. This particular variety of platinum Molly has a one-of-a-kind appearance and can grow to a maximum length of approximately 5 inches.

Golden Sailfin Molly

Breeders have produced a virtually infinite number of color and morphological variations as a direct result of the extensive use of these fish in aquariums. The Golden Sailfin Molly variant is easily identifiable due to its brilliant golden-orange color. This particular variety of Molly resembles a cross between a sailfin Molly and a goldfish in terms of its overall appearance. Both sexes have the potential to grow to a length of around 6 inches, but the males are significantly larger than the females.

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How Long Does It Take For Mollies To Reach Full Size?

Female Molly fish are often a bit larger than male Molly fish and can grow to be approximately 4 inches or 10 centimeters long, whilst male Molly fish typically reach maturity at a length of around 3 inches or 7 centimeters.

It will take the Molly fish about three to four months to reach their full size, the exact length of time for which will, in part, be determined by the water conditions, diet, and other factors. A molly fish should reach its full size in little more than four months if it is well cared for during its development.

How Important Is Tank Size For Mollies?

Mollies are very particular about the size of their tank. Even when housed in tanks of a similar size, these fish rarely engage in aggressive behavior with one another because of their generally docile nature. However, if you want them to develop well, be healthy, and be happy, you need to make sure that you give them a tank that is large enough for their needs.

The important thing for you to understand about this topic is that various species of Molly fish reach varying sizes as they mature, and as a result, they call for tanks of varying capacities. The smaller Short-finned Molly should do OK in a tank of 10 gallons, but the larger Sailfin Molly will require a tank of 30 gallons.

Because normal Mollies are not particularly large, you can keep three or four of them in a tank that is 20 gallons in capacity. As a result, it is clear that the answer to this question relies heavily on the variety of Molly you keep as well as the total number of fish in your collection.

How Many Mollies Should Be Kept Together?

It is advised that mollies be kept in groups of at least three or four at all times. Mollies are what are known as shoaling fish, which means that they are happiest when they are in the company of other fish of the same species. Even though it’s preferable to keep many mollies in the same tank, you shouldn’t cram too many fish in there because it could cause stress to the mollies. Due to the fact that a single molly fish needs approximately three gallons of water in order to swim freely, a tank that holds ten gallons of water can comfortably host a small group of three mollies. This is because of the fact that molly fish choose to live in groups.

You should also make sure to have a greater number of female mollies than male mollies. Male mollies have a tendency to be more aggressive than female mollies; thus, it is important to have more female mollies than male mollies. They are able to chase females in the hopes of mating with them, as well as other men, in the hopes of establishing their dominance over the other males. In order to limit the likelihood of fights breaking out in your tank, you should strive to keep a ratio of three female mollies to every one male molly.


How Many Mollies In A 10-Gallon Tank?

Three mollies can live in a 10-gallon tank of water. On average, you’ll need one gallon of water for every inch of fish you intend to keep. Mollies grown in captivity typically reach a length of 3-4.5 inches, so three gallons of water per fish is required for swimming freedom. If, however, your tank is part of a larger community tank, you should think about the needs of the other tank residents as well.

How Many Mollies In A 5-Gallon Tank?

Mollies are one of the most popular fish kept as pets because of their attractive colors and placid nature, making them ideal for inexperienced fish keepers. It is possible to care for one molly fish in a tank of only five gallons. Although a 5-gallon tank is a little on the small side for a molly, so this is never a good idea. Highly active mollies need plenty of room to swim around in.

How Many Mollies In A 20 Gallon Tank?

In a 20-gallon tank, you can keep up to four female mollies and two males (smaller species). Because of this, there won’t be as much fighting among the fish because each male will have access to two different females for mating purposes. This is due to the fact that the attention of the males will be split between the females, and there will be more room for everyone to swim around without feeling as though they are being crowded.

How Many Mollies In A 30-Gallon Tank?

You can put as many as four mollies into the tank, but it depends on which species you get. However, in order to keep sailfins and other larger mollies, you will require an aquarium with a capacity of at least a 30-gallon tank. Each additional molly will require approximately three additional gallons of water in order to live a comfortable life.

How Many Mollies In A 40 Gallon Tank?

According to the rules, you are allowed to keep one fish that is one inch long for every gallon of water. Therefore, if you intend to keep small fish (between 1 and 2 inches), such as Guppies or Mollies, you can keep anywhere from 15 to 20 fish in a tank that holds 40 gallons of water. Well, in a 40-gallon tank, it’s best to keep fish that are between 1-2 inches and 3-6 inches long.

How Many Mollies In A 55-Gallon Tank?

The rule of one inch per gallon is still in effect, so be sure that you do not put more than 55 inches worth of fish in a tank that holds a 55-gallon tank. The tank, with a capacity of 55 gallons, may house twelve mollies. You are, however, allowed to reduce the number in order to guarantee that they will not experience any discomfort at any point. It is preferable to have a few fewer than a few more than necessary because having a few more than necessary can really cause a lot of problems, ranging from cleanliness issues to overpopulation.

Which Fish Can Live With Mollies?

Even though they are often calm, mollies have the potential to become aggressive if they are housed in aquariums that are overcrowded or with fish that have a similar temperament. Because of this, it is essential to provide molly fish a lot of space to swim in and to only keep them with fish who are calm and unaggressive while housing them together. Mollies are social fish that love to be with other mollies, thus, it is best to hold off on adding additional fish to the aquarium until you have at least four mollies of their own species. 

Tank mates such as Guppies, Platys, Danios, Tetras, Gouramis, Swordtail, Angelfish, and even some Cichlids are great for Molly fish.


If you want to keep more than one molly in your tank, the first thing you need to think about is how big each individual molly is. Approximately 3-5 inches in length is considered to be the standard length of a molly fish. In accordance with the “one inch per gallon rule,” you will require a tank that is around 15 gallons in capacity to house three molly fish.

In addition to that, you need to consider the plants and the decorations that will be there. If you plan to keep other fish in your aquarium, you may want to keep fewer mollies than the guidelines suggest. Generally, the fish in the aquarium need to have the feeling that there is adequate space for them to swim freely about.

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