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Molly Fish: A Complete Care Guide For Healthy And Happy Fish

molly fish

The Molly Fish, also known as Short-Finned or Black Molly (Poecilia sphenops), is a tropical freshwater molly belonging to the Poeciliidae family.

They are very hardy fish and attract aquarists for their diverse colors, such as orange and black. In addition, other variations include Balloon and Veil Mollies, for example.

The species accepts a wide range of water parameters and remains at optimal size and behavior for community tanks. Mollies are commonly the first fish of many aquarists.

In short, they are hardy, easy to care fish that like to swim throughout the aquarium and should be kept in a group. This species is incredibly active, and when we mix different colors and patterns, we have a very colorful and fun aquarium

Molly Fish Overview

Scientific Name: Poecilia sphenops
Common names: Black Molly, Short Finned Molly
Distribution: North and Central America
Size: 4-5 inches
Life expectancy: Up to 5 years
Color: Fish can have different colors
Diet: Omnivorous herbivore
Temperament: Peaceful
Minimum tank size: 10 gallons
Care level: Easy
Breeding: Ovoviviparous


The Molly Fish is native to North and Central America, from Mexico to northern Venezuela and Colombia. Some populations are reported on certain islands in the Caribbean. Feral populations of this fish can be found in several countries across the globe.

The species was accidentally introduced through the release of individuals by inattentive aquarists or by fish that escaped from stores and breeding facilities.

These Mollies, in a natural environment, are found in shallow rivers, lakes, and estuaries, preferring low coastal areas with calm to medium-flowing water with the presence of marginal vegetations.
​The water on most biotopes can be characterized by crystal clear, tropical temperatures, high oxygen available, low CO2, low to medium salinity (hard and alkaline).

These fish have high resistance to salinity, making it possible to find them in environments with the concentration of salts, such as the mouths of rivers and mangroves.

In the places where they are introduced, they inhabit different types of environments such as mangroves, estuaries, lagoons, and lakes. It can even be found in environments with poor water quality.​

The environment in which these fish live is in constant variation (even daily) of different parameters, such as temperature, salinity and pH.


Adult Female Molly can easily grow to 5 inches in length when well cared for and fed. Males are smaller, around 4 to 4.5 inches. Some wild specimens can reach up to 8 inches.


In an aquarium with all the correct water parameters and an ideal diet, this animal can live for around four years, but the most common lifespan is around three years.

In nature, these animals are prone to live less, as they are predisposed to diseases, attacks from other animals, and environmental causes.

Molly Fish Appearance

Wild mollies are very different in color than their aquarium cousins, displaying a dull gray color typical of wild fish. Its fins and its body can present spots or spots in rows or slightly random formations, forming a specific pattern on the animal’s body.

Males ready to mate undergo a slight color change and may show a slightly bluish or greenish glow.
​These fish have the classic shape of livebearers. A stout, laterally flattened body, in symmetry with its height and a triangle-shaped head with a pointed mouth.

The ends of its fins are rounded, the tip of the dorsal fin is high and angled, and the tail is fan-shaped.

Its body structure is presented as being wider precisely in the middle of the animal, tapering towards the ends.

Molly fish have their mouths pointing up, indicating that they feed on the water’s surface.

Colors and Patterns

While there are numerous varieties of color and shape of this fish, most have a similar body shape and style to the typical Molly.

This species has several varieties with different colors and body conformations as an ornamental fish, giving rise to several subdivisions such as the Balloon and the Lyretail Molly.

Sexual Differences

There is apparent sexual dimorphism in adult animals.

Adult males are smaller and more colorful than the females. In addition to have an adapted anal fin in the form of a gonopodium (anal fin adapted for reproduction, in the shape of a hook).

Females are larger and more robust than males. Females do not own a gonopodium; instead, there is a typical anal fin.

Molly Fish Behavior

Molly Fish is a tropical freshwater species with a peaceful behavior towards other fish, because of this, it can be kept in a community aquarium with fish of the same size and behavior.

They are very active animals, occupying practically all strata of the tank; however, they prefer to feed on the surface. The Molly exhibits a very aggressive frenzy when feeding.

The fish behavior is more natural when kept in a group of at least three individuals. If you keep in pairs, there may be agressive persecution of the only female by the male.

These fish can be aggressive with each other when a more significant number of males is introduced, presenting a hierarchical dispute. Because of this, you must maintain the ratio of two or three females to each male.

Their behavior can change if they don’t feel comfortable in the tank hidden among vegetation, rocks, and driftwood.

While not necessarily aggressive to other fish species, some Mollies can occasionally nibble on slow-moving or long-finned fish. This behavior is more evident when space is limited or the fish are under stress.

Aggression can also occurs with similar-looking fish. When kept under ideal conditions, Mollies are calm and peaceful fish. When stressed or kept in small spaces, Mollies can become aggressive, attacking others fish.

Although very active, most of the action happen during the day, These fish like to rest at night or when the lights are off.

These fish swim through the entire water column and play and hide in the vegetation and among the decor. They will only stay hidden when feeling threatened or when something in the tank is wrong.

Molly Fish Care

Caring for your Molly Fish is an easy task. Mollies are hardy fish that can adapt to different aquariums and waters, as long as they tend towards the alkaline side, but a specific tank setup will ensure that the fish will thrive in captivity.

These fish in the wild have a varied diet that you should replicate in your home aquarium.

Molly Fish Tank Size

Although these Mollies do not reach a large size, this fish is highly active and must be kept in a group of at least three individuals, hence the need for an aquarium with reasonable space.

Therefore, they should inhabit aquariums over 21 gallons, with dimensions starting at 31″ x 11″ x 15″. This size accommodates a small group well, always remembering that the more space available, the better the welfare and development of the fish.

Molly Fish Tank Environment

As for any other aquatic animal, an aquarium heater and a filtering system are essential to keep the tank with Molly Fish healthy. The filtration system must be well-sized, creating a low to medium water flow.

Given the rusticity of the species, it can inhabit different aesthetic styles of tank. Even though aquarium decor is not critical to ensuring successful maintenance of the species, it is incredibly beneficial to have a tank that mimics the conditions of its biotope.

Densely planted aquariums with decorations such as driftwood and rocks provide shelter for fry and help dissipate aggression caused by hierarchical disputes within the group. It also provides a place for females give birth and to hide from the male’s continued harassment.

Decoration gives the aquarium a more natural and aesthetic peal, and the fish will be more comfortable under natural conditions.

As densely planted as the fish tank is, you should leave an open space for them to swim freely.
​These fish do well in well-lit environments and slightly shaded environments. They are indifferent to the substrate, make sure the tank is well decorated, and forms hiding places and territories.

Avoid keeping the species in a blackwater aquarium (with dark water due to the presence of tannins). As much as these fish may come to inhabit dark waters, the concentration of acids in aquariums may unbalance the salts present in the water.

In the same way, avoid using salt in the water of your Mollies aquarium; if you need to increase the hardness or pH, make use of substrates, rocks, and products suitable for this use in aquariums.

Molly is a fish-like any other that will generate organic matter through its food and feces. Then you must use a filter in the Molly Fish tank. Excess organic matter will make the water polluted, leading to illness or even death in the short term.

However,  the filter is necessary but it will not work any miracles. It is important to carry out its regular maintenance. Always measure the parameters with tests and make them perfect for your fish.

Water parameters

They are incredibly resistant fish, which support a wide range of parameters. These Mollies species can be susceptible to pollutants in the water, the accumulation of organic matter in the tank, and very acid and soft water.

The perfect temperature for keeping the Molly Fish in an aquarium is 64 to 82 F. The ideal pH range is between 7.4 to 8.6, and hardness is 11 – 30.
​It is a great fish option for beginner aquarists due to its rusticity.

This difference between hardness and pH is related to the region of origin of the specimen and demonstrates the oscillation of parameters during the dry and rainy seasons.


They are remarkably disease-resistant fish when kept in pristine condition and a stress-free environment. Your fish should have no problems by keeping the water quality and the tank always in excellent condition and with high-quality food.

As this is not always easy to achieve, we see individuals with susceptibility to stress-related diseases, such as Ich, and signs in different tanks of infections such as Fin Rot, Popeye, etc.

They often suffer from external conditions, such as the toxicity of nitrogenous nutrients in the water (such as ammonia and nitrate).

But the most important to know is, like most hard and alkaline water fish, this Molly is highly prone to acidosis.

This condition is seen when the pH or hardness of the aquarium water is below the ideal range required by the species in question. This disease does not necessarily affect all aquarium inhabitants because fish of different species differ from each other in the field of pH values that are acceptable to them.

The main symptoms are, the fish can lose mobility, and appetite, increase mucous secretion, and its gills can change color. Hemoglobin can be affected, causing the animal not to breathe correctly and become disoriented, gaping at the surface, or, if the change is too extreme, causing its death.

In more advanced cases, the fish may have corrupted ends of the fins, white spots on the body, closed fins, lack of appetite, and stress.

This condition can be acute (when it occurs because of a sudden change) or chronic (when the change happens over time). Carrying out weekly tests of this parameter will be essential to have a balanced aquatic ecosystem.

Molly Fish Tank Mates

They are peaceful fish and can coexist without problems with other animals; they can live with countless other fish species in a community aquarium. While very friendly, some mollies can become highly aggressive, relentlessly attacking other fish.

Individuals with more aggression should be kept isolated or in high-volume aquariums with many Mollies of the same species.

Avoid aggressive and larger species that could injure the Molly fish, or that will compete directly for food. Likewise, smaller and fragile fish should be kept with caution, as they can suffer aggression.
​Species that are slow to feed should be avoided, as Mollies demonstrate a very energetic feeding frenzy. It is best to keep them with fish of similar size and behavior.

This species may bite fish with very flashy and exuberant ends, such as Bettas and Guppies; in this case, avoid using fish with these characteristics.

This species can hybridize with other Mollies and even other livebearer species.

Corydoras, Zebras Pleco, Tigers Pleco, Bristlenose Pleco, Cherry Barbs, Zebra Danio, Platys, hard water Tetras, and livebearers like Swordtails make great tank mates.

The Molly Fish is often kept as a single species in large aquariums, planted tanks, or biotope-type aquariums where the species’ original location is mimicked.

It is a great species to be kept alone in a well-decorated mono species tank. A group forms many beautiful movements and a beautiful color bright tone contrast.

Molly Fish Food And Diet

This species, like other Mollies, is omnivorous with a high tendency toward herbivorous, feeding on anything that will fit in its mouth.

In the wild, the Black Molly’s primary diet is various foods, including algae, detritus, and worms. Its consumption will depend on the availability of food in the natural habitat.

In an aquarium, these fish readily accept all types of food, balancing the diet with the food supply for herbivores.

A good combination of using a high-quality commercial feed and live, fresh or frozen foods such as brine shrimp or daphnia, with fresh or bleached vegetables like zucchini and cucumber will make these fish look much more vivid and their behavior closer to natural.

Set a time to feed the fish, always trying to give an amount that can be consumed quickly. They are fish that are always hungry. Try to feed your fish a varied, high-quality diet. Providing a diverse and balanced diet is essential for the species to demonstrate its full potential.

How To Breed Molly Fish In Aquarium?

Captive breeding is very easy and is carried out commercially and even accidentally in home aquariums worldwide.

It is an ovoviviparous species, where the male must internally fertilize the female. The eggs develop inside the mother until they hatch, and the fully developed offspring are born.

The female simply disperses the fry somewhere in the water column.

Females can store sperm from the males to fertilize later and produce subsequent litters. After the gestation period, which can vary between three and four weeks, females birth to fully formed offspring (about 20 to 150).

The species exhibit no parental care. They are incredibly prolific and reproduce quickly.
​Throughout the gestation period, the molly will show several behavioral changes. The behavior change will begin right before the courtship and increase as approaches the birth of the fry.

The most unmistakable sign in the female is the gravid spot, a black spot near the anal fin. This spot gets darker as the female gets closer to giving birth.

Also, when they are close to giving birth, the females hide in different places in the aquarium, avoiding contact with other fish. Ensure there are good decorations and plants so the females and the fry can hide them during this time.

If you want to raise the fry, you can separate the female in a breeding aquarium and wait for the spawn. However, females can eat fry. Because of this, there are different types of incubators/hatcheries to separate the fry from the mother.

Usually, infusoria, water fleas (Daphnia sp. and Moina sp.), brine shrimp nauplii and comercial feeding are used as the first food for the fry.

The choice of artificial feed for the fry should be selected for their size to fit in their tiny mouths. Feeds of different sizes are used at various stages of development to ensure good health and survival of all the baby Molly.

Are Mollies good beginner fish?

These fish have been popular for years, and that’s no wonder, as they’re great fish for beginners.
​They are very resistant animals, cheap and easy to find. They are undemanding and very active in the aquarium, always ready to eat on the surface with their large mouth facing upwards, or eating algae on plants and decorations.

They have no problems getting along with other fish because they are very peaceful, and everything is available in a multitude of colors and shapes. This all makes it the perfect choice for both novice and experienced aquarists.

If you have read and understood the main requirements in keeping Molly fish, likely, you will no longer go wrong when going to a pet shop and buying your fish.


Molly fish are peaceful, hardy, and social, making them perfect aquarium fish. They can be kept single, but they prefer to live in groups.