Molly Fish – A Complete Guide – Care, Diet, Facts

Molly fish, or mollies, are fish that many like to keep in their home’s aquarium.

It’s necessary to understand what a molly fish needs before adding them to your tank, however. While they’re a great fish for new aquarists, it’s important to research their care requirements before getting one. It’s with the confidence of this knowledge that you can bring these fun fish into your house.

Research must be done prior to getting your fish. To be a good molly fish owner, you need to understand how to provide them with the proper aquarium set up, dietary needs, and more.


About Molly Fish

Mollies are fish that live in the fresh waters of the Americas. This widespread range of habitats make them easy to care for with regards to water conditions.

Molly fish belong to the Poeciliidae family of fish.[1] Many popular household fish are members of this family, including mollies and guppies.

Naturalist Charles Alexandre Lesueur, a French Ichthyologist, was the first to discover mollies. An Ichthyologist is a fish scientist. Lesueur first discovered sailfin mollies in Indiana.[2]

Since their discovery in 1821, we have learned of almost twenty molly fish species. All of which vary in size, color, and shape.


Many people choose to place molly fish to their aquarium as they’re an interesting, attractive addition. With supply for these unique, active animals, also comes demand. The practice of breeding mollies for aquariums has varied their appearance. Through specific breeding practices, mollies’ appearance has become even more diverse.

With almost 20 kinds of mollies, they come in many colors, sizes, and even shapes. This range of appearances is one of the reasons mollies are so popular among fish keepers.

As a member of the Poecilia genus, they are one of 40 species in the genus. 39 of these species are mollies. They come in many colors, sizes, and even shapes. This notable range of appearances is one of the reasons mollies are so popular among fish keepers.


Molly fish come in a range of colors from black to white. The Black Molly Fish, known for the unique dark hue over their entire body, is a common species.

A trait that many fish, including mollies, have is a shiny, metallic appearance to their scales. This iridescence is created by iridophores, a cell that refracts light.


Molly fish have been seen in a variety of colors and patterns that are unique to each individual in the species. Whether it’s stripes, spots, or blotches, each molly fish has a pattern that differentiates them from one another.

It is also believed that these patterns seen on mollies’ bodies and tails contribute to natural or sexual selection.

The subspecies of mollies are often distinguished by their patterns. Dalmation mollies, for instance, have black and white spotting like their canine counterparts. Balloon mollies, another popular species, often have stripes through their tails or bodies.

Tail Shape

Despite their small size, mollies can have long, attractive tails.

Sailfin mollies, best known for their long, colorful dorsal fins, are a common species. The dorsal fin is the fin on the top of the fish’s body. A sailfin molly’s dorsal resembles the larger, sailfish.


The molly fish is a dimorphic species. Being a dimorphic species means that the males and females can be distinguished based on differences to their appearances.

In their natural habitat, females tend to be larger. Females average size is 4.5 inches. Males average size is 3 inches.

Males have a pointier anal fin, or the middle fin on the fish’s underside. Females’ anal fin fans out.

These dimorphic traits allow owners of home aquariums the ability to identify their mollies’ sex with ease.

Tank Requirements

Before bringing your molly fish home, you need to provide them with the proper home aquarium. The first stage is setting up their habitat.

When buying a tank, ten gallons is the smallest appropriate size. For every ten gallons, you can house four common molly fish. This is with the common molly fish in mind. Larger species of mollies will need larger tanks.

Before your fish are introduced, you need to cycle your tank’s water to allow in good bacteria.

Mollies can live in a variety of habitats. Often found in shallow rivers in South and North America, they can adapt to many conditions. You might need a heater for your aquarium to match these natural conditions. Their water should always be between 72 and 78 degrees.

To ensure their tank meets these requirements, you should have a thermometer, as well. The thermometer should be placed away from the heater so that it can get a proper read. This will help ensure that you have an accurate reading of your tank’s temperature.

You must measure pH in your home aquarium. PH, or potential of hydrogen, is the measure of acidity in the aquarium. Mollies do best in pH levels between 6.5 and 8.2.

You should maintain a tank’s cleanliness through routine cleaning and water filtration.

You will need a filter for your tank that is appropriate for your tank’s size and needs.

Furthermore, Mollies should have a lot of vegetation in their tanks. They’re accustomed to habitats they can hide in as necessary. Plants and decorations that provide shelter are best for mollies.

Social Behavior

Mollies are peaceful but can show aggression at times. This is often due to stress caused by overcrowding, aggressive tank mates, or poor care. A home aquarist can prevent this with proper care and maintenance.

Molly fish should not live with animals that they might nip at or eat. This includes fish that are smaller than their mouth and fish with long fins or tails that mollies may bite.

If you have males and females in your tank, you might see courtship behavior.

Mollies are social fish that prefer living in schools. These schools should consist of more females than males. Males can harass females, which can cause stress.

If your molly fish is less active, it could be due to illness. Behavior should be monitored accordingly due to this.

Keep in mind that mollies can live up to five years in a household aquarium with good care.


Mollies are ovoviviparous fish which means that the eggs hatch inside of the female’s body. The molly fish then gives live birth to the young.

It is easy to identify the males and females. To reiterate, females tend to be larger than the males.

When breeding mollies, warmer temperature is optimal. To induce breeding, you can raise the water temperature, but to no higher than 78 degrees.

The males will actively move around the female and she will determine if he can fertilize her eggs. Females have shown preference to male mollies with larger dorsal fins, or the fin on top of their body.[3]

Mollies’ gestation period is between 35 and 45 days. Pregnant mollies are easily distinguished by their swollen abdomens.

Mollies can give birth to up to 100 live fry, or young fish. These fry resemble smaller versions of the adults.

Removing the adults from the young is important. Many adult mollies will attempt to eat the young.

Many choose to use a breeding tank, but fish should be monitored during transport as it can be stressful.


Mollies are an omnivorous fish, meaning that they eat both plants and animals.

Mollies will scrape algae from surfaces for nutrients. Owners should still provide a varied diet, as well.

Your mollies will require a high-quality fish food that is focused on providing your fish with a lot of protein. Check the ingredients to ensure that the first few items are high in protein, like fish or shrimp.

It’s important to avoid food whose first ingredients are wheat or soy. The first ingredients should be something higher in protein.

Your molly fish can be fed more organic protein and fiber as well in the form of bloodworms, shrimp, or vegetables.

Providing your mollies with a variety of dietary needs is important to maintain good health.

It’s important to be conscious of how much you’re feeding your mollies. Molly fish should only be fed twice per day. When fed, they should only be fed for two minutes at each meal. Overfeeding your molly fish can lead to health and maintenance issues. In some fish, overconsumption has been known to be fatal.

If you are setting up a breeding program for your mollies, you should be prepared to feed their fry small amounts of food, more often.


Molly fish are a tough species, but are susceptible to disease.

One infection seen in mollies, called “molly disease”, is common in mollies. Also known as “shimmies” or “livebearer disease”. This condition is due to poor water conditions. Symptoms include fish being less active and staying in one spot.

“Ich” is another disease common to molly fish. You might see your fish scratching against the side of the tank. This illness can also cause small, white dots to grow on your fish.

It’s important to understand your fish’s behavior to be informed when your fish is acting abnormal. This abnormal behavior, such as lethargy or eating less, can only be noted by understanding your fish’s natural behavior. When you notice an illness early, it is often possible to put them on medications or other treatments to avoid worsening or even treat the situation.

It is important to try and avoid these fungal infections by providing a healthy environment and diet. A healthy environment has both physical and mental requirements.. Physical environments include providing a clean environment through water filtration and routine cleaning. Mental environments include keeping your fish’s stress levels low through having tank mates that get along well and avoiding overcrowding.

Global Reputation

Humans have used mollies’ traits to scavenge in practical ways. Black mollies introduction to areas suffering overwhelming pest populations has proved useful.[4] Mollies and their fellow Poeciliidae, guppies, eat mosquito larvae. This helps control the mosquito population in ravished areas. In turn, this helps with diseases like malaria. This practice has become popular and utilizes several species of fish.[5]

Unfortunately, introducing mollies to habitats can also cause overcompetition for food. This harms other fish populations with similar diets in areas of introduction. This practice has benefited pest populations. However, it affects aquatic ecosystems negatively.

Unique Traits

Molly fish have a unique adaptation. Adaptations are traits that better allow an animal to thrive in their environment. Mollies have adapted to eating algae through the shape of their head and mouth.

The mollies’ head is flat with a protruding jaw. This jaw serves as a scraping tool to better grab the algae they eat.[6]

As scavengers, molly fish are one of several species that assist with keeping their environment clean. By eating algae, mollies can do the same for your home aquarium. This is another practical reason many have them in their household aquariums.

At least one species of molly fish, the amazon molly fish, is asexual.[7] Asexuality, or the ability to reproduce as an individual, is rare in the animal kingdom.

Asexual animals, like the amazon molly fish, essentially clone themselves. Sexual reproduction consists of two sets of DNA. Asexual reproduction consists of one set of DNA.

There are downsides to this process. Asexuality makes it difficult for genes to pass down.Likewise, asexuality is not a sustainable process as no new DNA is being introduced to these offspring. Despite this, the amazon molly flourished for thousands of years.

Are Molly Fish Right For Me?

Molly fish are great fish for both beginners and experienced fish keepers alike.

The instructions listed in this article are provided to ensure that you are prepared to be a great aquarist to your molly fish.

As active, colorful fish, you are one your way to having a home tank full of molly fish.

While it’s important that you have the proper information, tank, and good for your molly fish, with the right preparation you are on your way to being a great fish owner!








Leave a Comment