Black Molly Fish – Care, Diet, Breeding, Tank Mates

Black mollies get their name from their dark bodies, which is a result of a severe case of melanism. They are born with a lack of color pigmentation, although some of them display a yellow streak running from the dorsal fin.

It is hard to locate the black molly fish in the wild, primarily because it has gotten its signature characteristics from captive breeding. The black molly is indigenous to Central and South America, where they prefer freshwater habitats. The species is also adaptable to salty water environments.

Black Molly Fish

Black Molly Fish

Housing Black Molly Fish

Black mollies should be reared in large tanks that encourage stable water conditions. Since mollies also love to roam, you should provide at least a 20-gallon tank for a small number of the fish.

You will need a tank of 45 gallons if you want to rear the fry to a significant size.

Black mollies thrive in freshwater aquariums, although they can adapt to any environment. While it is commonly thought that the species are salt-water fish, their natural habitat is actually freshwater.

An efficient filtration system should serve the tank that you keep the black mollies. Frequent water changes are necessary to ensure that there are no levels of ammonia and nitrates that can harm the aquatic pets.

Black mollies will survive in different varieties of lights, but it is best to expose them to sunlight. Artificial lights can also be used to keep the mollies and the plants healthy. Black mollies are mostly mid and top-dwellers, so the kind of substrate you choose should be ideal for the plants in the tank.

Feeding Black Molly Fish

In the wild, black mollies sustain their omnivorous diet with small invertebrates, plant matter, and algae. They will, therefore, appreciate a varied diet of vegetables and high-quality live food. Black mollies will consume all green vegetables, although fish owners will mostly feed them cucumbers, shelled peas, and zucchini. You can provide these vegetables once or twice a week.

Flake food should be supplemented by such live or frozen food like brine shrimp, daphnia, bloodworms, tubifex, and blackworms.

Black mollies will also eat any algae in your tank, but you can also offer them algae wafers as part of their diet. It is advisable to feed them twice or thrice in a day, although you should refrain from overfeeding them. Give the mollies enough food to finish in three to five minutes.

Breeding Black Molly Fish

It is quite easy to breed black mollies, provided you encourage a comfortable environment in the tank. The first thing to observe is the male to female ratio if you want successful breeding. You can keep three to four females to one male since male black mollies can become aggressive during mating.

During copulation, the male will release a bundle of sperm into the female’s vent via his gonopodium. The gestation period is around seven to eight weeks, after which the female delivers fully-formed fry. You can tell the females are pregnant if you observe a substantially plump stomach and a dark gravid spot close to the anal fin.

At this point, most aquarists will move the pregnant females to a separate tank. Male mollies will often chase the females around and stress their pregnancy. Mollies are also known to cannibalize their young ones, and moving the females will encourage more fry to survive. If you decide to keep the adults and dry in one aquarium, provide a lot of live plants for the fry to hide.

You can expect between 40 to 100 fry for every birthing process, although the age of the parents will primarily inform the number the babies. Baby black mollies do not need any special care, and they can consume flake food from the get-go.

You can feed them micro-worms, baby brine shrimp, and daphnia. The fry should be allocated food three to four times a day to encourage full development. You can keep the babies in a separate aquarium, where you can feed them multiple times a day until they reach a size where they cannot be fed on by the adults.

It typically takes around two months to tell the females and males apart. You can opt to separate them at this time to discourage breeding between siblings.

Black Molly Male to Female Ratio

It is vital to consider the male to female ratio in an aquarium with black mollies. There should be two to three females for every male.

Male black mollies are known to relentlessly pursue females for mating, which can stress the females in a tank with multiple males. It is easy for the females to get stressed from the harassment and fall ill or even die.

Black Molly Fish Tank Fish Mates

Black mollies thrive in community tanks with other peaceful fish species. You should not rear them together with bigger fish that might mistake them for prey. Some tankmates to consider, include:


Guppies are an excellent beginner species with a relatively peaceful temperament. Guppies will brighten up your tank with their lively colors, with the males being more brightly-colored than females. Guppies grow to a maximum length of 2.5 inches and, like mollies, are active swimmers.


Gouramis are common aquarium fish, thanks to their impressive hardiness. They have adapted lung-like organs that enable them to breathe at the surface of the water. This organ helps gouramis to thrive in community tanks. It is advisable to source for the dwarf varieties as they are known to be more peaceful.


If you want to maintain a lively tank, you should consider keeping platies. They are available in an assortment of colors and grow to between 1.5 to 2.5 inches. They also share a similar diet with black mollies, making them perfect tankmates.


Tetras are quite social, although they are more popular for their flashy neon colors. If you are going to include another species, avoid long-finned varieties because tetras are fin-nippers. Tetras are also omnivores, which will make it easier to feed them together with black mollies.


You can choose between the many varieties of the danios fish, including the Zebra danios. Danios are known for the horizontal stripes across their bodies and their eye-catching colorations. Danios should be reared in schools and are active maters.

Black Molly Fish Lifespan

If cared for properly, black mollies will live for up to five years. Young ones mature at four months, after which they are ready for mating.

Ideal Water Temperatures of Black Mollies

While black mollies will survive in a variety of water conditions, they are best kept in freshwater conditions. You also need a bigger aquarium to accommodate the active nature of the fish.

Other parameters to keep in mind include:


Black mollies are a tropical species, which means that they will thrive in warm temperatures. The ideal range is between 70 to 80ºF, which can be maintained by using a heater.

The temperatures should also be stable to avoid getting your black molly vulnerable to stress or illnesses.

Water pH & Hardness

When it comes to water pH, black mollies prefer alkaline and harder water. Ensure the range is between 7.5 to 8.2 to ensure that they are comfortable. The hardness should be between dH 20-30.

Water salinity

There is an ongoing debate on the necessity of salt in aquariums with black mollies. There is no need to add salt if your tank has alkaline, hard water. The addition of salt has, however, been shown to have positive effects if black mollies are not reared in optimal water conditions.

Common Black Molly Diseases

Black mollies can easily get ill in poor water quality or if they are stressed. Some common ailments to look out for include:

Ich / Ick

This condition manifests as white spots on the fins and skin of your pet. The black molly can be seen rubbing its skin on objects to try and relieve the discomfort on their skin. Besides poor water quality, ich can be introduced by new fish that are not quarantined before being added to the tank.

Aquarists will commonly gradually raise the temperature to encourage the parasite to slide off the fish. A major water change should be done afterward, in addition to cleaning the substrate.

Fin / Tail Rot

This ailment mainly affects stressed fish, so you should take care not to couple your black mollies with aggressive fish. Bacterial and fungal infections also cause fin and tail rot, where these organs appear frayed, shredded, or torn.

You can also notice a white sheen at the edges of your pet’s tail and fins due to the bacterial presence. It is best to quarantine the affected mollies and provide them with fungal antibiotics.


This condition is triggered by the parasite Oodinium, which will bore into the skin of the fish. If you are keen, you can see hold-colored cysts on the skin of your molly.

The cysts grow larger over time, and it can be hard to detect the disease in its early stages. Once it advances, you will see more noticeable lesions on the skin. If detected early, velvet can be addressed with copper medication.

Fish suffering from velvet will appear clamped and will struggle to breathe. Since it is very infectious, it is recommended to quarantine the sick fish and perform a 100% water change.


Dropsy is quite common in modern aquariums. It mostly attacks species with a compromised immune system from stress or poor tank conditions. The most notable symptom is a swollen stomach because of the accumulation of fluids. The disease will subsequently spread to other organs and cause the death of your black mollies.

In addition to being hard to treat, dropsy also spreads quickly and will infect all of the tank’s inhabitants in no time. Once you suspect the presence of the illness therefore, move the affected fish to quarantine and treat the tank by making water changes.

Anchor Worms

Anchor worms are crustaceans that burrow their way into the skin of a fish and target the muscles. The area where the worms bore into the skin becomes red and irritated.

You can also spot teeny hair-attachments on your molly that appear green-colored. The worms release eggs in the body of the fish and cause significant damage over time.

The affected fish can be seen scratching the affected areas against objects. Some aquarists will opt to physically extract the parasites and clean the inflamed areas with an antiseptic.

You can also bathe your black molly in a seawater solution for around five minutes and several days until the parasites drop off.


Another parasitic infection that is common with black mollies involves body flukes. The parasites are common in tanks with poor water quality or in overcrowded situations.

While the flukes are often present in aquariums, they remain harmless under appropriate conditions. Do not keep your mollies with incompatible species as it may stress them and make it easier to be affected by flukes.

Black mollies with flukes are typically observed gasping for air at the tank’s surface or bottom. You can also treat the aquarium with special aquarium medication.

Mouth Fungus

Mouth fungus is typically associated with the mouth of a fish, although it also affects the gills, face, and even the body. The condition is particularly observed among stressed fish, such as those that live with aggressive species.

If an area of the body is injured, the fungus will invade and cause an infection. The infection commonly appears as white splashes or threads on the body or mouth of mollies. Antibiotic treatments are effective in this case, as is aquarium salt in controlled amounts.

Do Black Mollies Need a Filter?

Black mollies are vulnerable to poor water conditions, which makes an excellent filter a necessity. The species are known to eat quite a lot, and a filter will clean up the subsequent waste. Uneaten food can quickly lead to a spike in ammonia levels, leaving your mollies stressed or sick.

Most filters will function well in an aquarium with black mollies, and the choice will thus depend on preferences. You can use a hang on back filter because it will not disrupt the water flow too much.

While a canister filter is often praised for its high-efficiency, it can be overkill with the fish. Remember to use an internal filter that is safe for fry if you are going to raise the young ones in the same adult tank.

Do Black Mollies Need a Heater?

Black mollies are tropical fish, and they thus prefer warmer water in the range of 70 to 80ºF. The fish also thrive in stable water temperatures, and sudden changes will affect their health.

An aquarium heater ranks as the most effective method of maintaining stable temperatures in a fish tank. Choosing a heater will require you to consider the size of your aquarium. Aquarists will commonly calculate five watts per gallon, which means that you will need a 125-watt heat for a 25-gallon tank.

Modern heaters are fitted with thermostats to prevent changes in the room temperature from affecting the aquarium temperatures.

You can do without a heater if you reside in a tropical area, but you will still need one in a well-heated household. If you want to keep black mollies outdoors, they can do without a heater in tropical climates, but they will not survive through winter.

Do Black Mollies Need Live Aquatic Plants?

Black mollies appreciate planted tanks, and they will feel very comfortable among a lot of plants. Plants are especially vital to boost the survival rate of fry. Black mollies can eat their babies, and the young ones need to have spaces to hide.

If you opt to have floating plants, ensure that the mollies can roam the surface for food. Some plants to consider include java ferns, java moss, hygrophila, and amazon sword. You can also incorporate decorations to add more caves for the mollies to hide in.

How to Tell if a Black Molly is Pregnant?

It can be harder to tell if a black molly is pregnant compared to other kinds of colored mollies because of their black bodies.

You can, however, observe the females for any changes in behavior. Check if she is swimming slower, consuming more food, or keeping to herself more. You can also check if there is a dark triangular spot close to the anal vent.

The stomach will subsequently start getting bigger and plumper as the pregnancy advances. The spot near the anal vent will also get darker and become completely black when the female molly is about to give birth. This should be a time when you move her if you intend to rear the fry in a separate tank.


While it’s thought that black mollies are hardy, the fish are actually very sensitive to water parameters. To start, they prefer freshwater and not brackish water, as is often thought.

Be careful of the male to female ratio when it comes to black mollies, mainly because the males are aggressive, and they can stress the females out. Provide them with a lot of space to swim and roam and decorate it with lots of live plants.

You can keep black mollies with similarly-sized fish like guppies, tetras, danios, gouramis, and female bettas. Black mollies are also susceptible to common ailments of freshwater fish, particularly if the water quality is poor.

Molly Fish   Updated: January 6, 2020
avatar Hello, my name is Fabian, and I am the Chief Editor at Aquarium Nexus. I have over 20 years of experience in keeping and breeding fish. The aquarium hobby brings me immense joy, and I take great pleasure in sharing my experiences with others.

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