If you’ve spent any time in a pet store looking at fish, you probably recognize the black molly. They’re a small black fish with a distinctive shape.
While mollies come in both distinctly different colors and physical types, this article will focus solely on the shortfin black molly.
Originating from South America, these are a tropical fish that thrive in salted freshwater. They can, however, live in a saltwater setup as well.
Prolific and easy to care for, they are a popular, classic choice for an aquarium.
Whether you’re looking to set up your first aquarium or have been keeping fish for years, the black molly can be a great addition to your tank.
In this article, you’ll learn everything you need to know about black molly care—tank size, setup, maintenance, and preferred community members—so your venture into keeping them will start with the greatest chance of success.
Overview and Statistics
In this table, we summarize the most essential information and care tips.
|Common name(s):||Black molly, molly, Mexican molly, pointed-mouth molly, Yucatan molly|
|Scientific Name(s):||Poecilia sphenops, Mollienisia sphenops, Poecilia thermalis, Gambusia modesta, Poecilia velifera, Poecilia mexicana, Poecilia latipinna|
|Origin:||Central and South America|
|Adult Size:||3 inches|
|Minimum Tank Size:||30 gallons|
|Typical Tank Setup:||Brackish water, heavily planted with strong filtration|
|Tank Level:||Middle–upper level|
|Water Conditions:||Freshwater (with added aquarium salt) or saltwater, 68-82° F, KH 10-25, pH 7.0-7.8|
|Tank mates / Compatibility:||Black mollies are peaceful and do well with other community fish capable of withstanding similar water conditions.|
Black mollies are short finned. Though there are balloon, sailfin, and lyretail varieties in black, we’ll be looking only at the shortfin black molly.
They are a hybrid variety, derived from the sailfin molly. They are well-known and adored aquarium fish. They do well in groups and are not aggressive.
They can occupy either freshwater or saltwater tanks. For optimal health in a freshwater aquarium, it’s recommended that you add one teaspoon of aquarium salt per gallon of water. From Central and South America, they can do well even in very warm waters.
Though small, they love to eat, and they produce a significant amount of waste. Therefore maintaining a healthy tank will require the use of a strong and efficient filtration system.
Unlike other species, the black molly bears live young, and will likely require your involvement for successful breeding in a home aquarium environment.
They are an attractive, streamlined fish. It is a melanistic breed and will be entirely black—though there may occasionally be some color leakage. These fish are relatively small, with full-grown males reaching only three inches in length.
The male black molly has a pointed anal fin and a significantly larger dorsal fin than the female. It may be difficult for a new fish keeper to tell male and female apart when looking at them individually. It’s usually readily apparent when you are looking at a male and female side by side, though.
Behavior and Temperament
They are a peaceful species. They do well in both community tanks and if living with only others of their kind.
Males can show some aggression when it comes to females and breeding. For the happiest fish, it’s recommended that you have at least two females to every male and provide the females with plenty of plants and hiding spaces.
Habitat and Tank Requirements
You’ll want to take the time to create an optimal environment where your black mollies can truly thrive. Here’s how.
What Size Tank do Black Mollies Need?
They prefer to live in large tanks, especially if they’re being kept with many fish. To provide the best care possible, you’ll want to make sure they have at least 30 gallons to call home.
Ideal Water Type and Parameters
The best water conditions be confusing at first because they can be kept in either freshwater or saltwater tanks. Of course, the kind of environment you choose will influence the other species you’re able to keep alongside.
If you do decide to house your black mollies in a freshwater tank, you should be prepared to add some salt. They do best in brackish water. Adding one teaspoon of salt for every gallon of water can give them an environment they’ll love.
When adding salt to an aquarium, always make sure you’re using aquarium salt and not table salt. You’ll also need to be sure the other freshwater fish in your tank can tolerate salt in the water.
Black mollies do best with a KH level between 10 and 25, and a pH level between 7.0 and 7.8. They do quite well in warmer waters, and the temperature of the water should be kept between 68 and 82 degrees.
They are particularly sensitive to nitrates—keeping many plants growing in your tank requires a close eye on your water’s parameters as abrupt changes can wreak havoc on your community.
What Substrate do They Need?
While they are highly adaptable and can be comfortable with many different types of substrate, crushed coral is an excellent choice for them.
Crushed coral will serve to provide extra calcium and increase the water hardness for your fish. This is especially beneficial to livebearers like black mollies.
While black they are a relatively smaller species, they are voracious eaters and, as a result, are quite dirty as well. To keep a tank with them in healthy, you’ll need to invest in a heavy duty and effective filtration system.
A canister filter is a great choice, though a hang-on-the-back filter will work as well—depending on the size of your tank and the other fish involved.
Black mollies spend much of their time cruising the surface looking for food, so it’s best that the filter you use leaves the surface waters as undisturbed as possible.
While mollies can adapt well to many different kinds of light, they benefit from having live plants in their tank. This means, for optimal growth and health, they should have access to sunlight.
Should you not be able to provide your tank with adequate sunlight, you can supplement with artificial light. Plan on allowing one and a half to five watts of light per gallon of water in your tank. This will help keep your plants—and in turn, your fish—healthy.
Plants, Decorations, Swims, And Open Spaces
Black mollies do best when they have plenty of plants in with them. Not only do these plants help make the adults feel more comfortable and experience less stress, but they also provide many places for the young to hide. As new fry are frequently targeted for meals, this is very important.
Floating plants can be a good option, but you’ll want to make sure your fish can still reach the surface to search for food. Java ferns, Amazon swords, java moss, and hygrophila are all great plant options for a black molly tank.
Similar hiding places can be created with tank decorations. The more nooks and crannies you can add to your space, the better.
Males can sometimes become territorial and aggressive, so allowing them to stake a claim on certain spaces while also giving other fish escape routes is a good idea.
How Many Black Mollies Per Gallon Should I Have?
All successful fish keeping begins with choosing an appropriately sized tank. The minimum needs for a black molly tank is 30 gallons. This is based not so much on their size but on their waste load.
Keeping them in a smaller tank will quickly lead to poor water conditions, even with very regular water changes.
In a 30-gallon tank, you can have a small school. They do best with at least three females to every one male. In a 30-gallon tank, you could easily house two males and six females if there are no other fish to account for.
With their waste load, it’s best to stick to this ratio, no matter how large the tank is.
Diet and Feeding
A good diet is essential to having healthy fish. They are omnivores, so they will require both plant and meat food sources. Let’s take a closer look at their natural diet, and what you should feed them at home.
What do They Eat in the Wild?
In the wild, they eat algae and plant matter. They also eat small invertebrates. As omnivores, they can eat almost anything they come across—especially if they’re hungry enough.
What Can They Be Fed in an Aquarium?
They do well on commercial algae-based flake food. For best results, your black molly feeding routine should include protein as well. Freeze dried brine shrimp, bloodworms, and tubifex can provide additional protein and be used to supplement their diets.
They like to search the surface for food, but they also scout the entire tank. Feel free to vary the feeding experience by alternating between slow and fast sinking food sources.
What Human Foods Can They Eat?
Vegetables can be a great table food to share with them. These can add some needed fiber to your fish’s diet and keep the feeding experience exciting—much like eating in the wild would be for them.
If you’re looking for appropriate vegetables, zucchini, cucumber, mashed peas, and spinach can all be fun to try. Remember to remove any leftover fresh food after 24 hours. It likely won’t be eaten and will only contribute to making your tank dirty.
How Often Should They Be Fed?
You should feed them once or twice a day. You’ll want to make sure any flaked food is crumbled for easy eating. Take care not to overfeed them. Black mollies love to eat—it even influences their choice in mates. Even if they’re full, they will continue to seek out the fresh food.
Once they start spitting out the flakes, you’ll know your fish are full. Those extra uneaten flakes will just make their way to the bottom of the tank and create a dirtier environment.
Any Special Care Requirements?
They need a powerful filtration system in place, together with regular water changes. Though hardy and able to adapt slowly to different kinds of water parameters, they can be sensitive to abrupt water changes.
Keep an eye on how dirty the tank becomes, as this can quickly alter the pH of the water. Your black mollies may not tolerate the change well and can be lost.
You’ll need to pay close attention to your male-to-female ratio. Males can be overzealous with their attention to the females. If not managed well, your poor females can be hounded to death.
Should you opt to have both males and females in the same tank, you’ll want to plan for any offspring. Black mollies are easy (and frequent) breeders. While fun to watch and experience, you’ll need to account for the fish and waste load that comes with new fry.
Compatibility With Other Fish
Black mollies do well with other peaceful species. However, you shouldn’t keep them with species significantly larger than they are. They are frequently used as feeder fish, and you don’t want to lose any to some hungry tank mates accidentally.
Swordtails, corydoras catfish, angelfish, platies, plecos, large tetras, black skirts, red serpaes, and silver tips all do very well in tanks with black mollies. Though they do have specific requirements, so if you’re catering to them, you should select companion species who enjoy the same kind of environment.
If you are integrating black mollies into an existing tank that requires different parameters from that which they’re coming from, it’s best to house them separately and slowly transition the water. Once the water they’re in closely matches the tank they’re heading to, you can safely introduce your black mollies to their new home.
Can Multiple Black Mollies Be Kept Together?
Yes. They are happy to live in a community together. To provide the best black molly care, you will only need to make sure your tank is big enough, your filtration system is strong enough, and your ratio of males to females is appropriate.
Black mollies can also be kept with other types of molly. If you’ve been eyeing the Dalmatian mollies or dramatic fins of the sailfin molly, there’s no need to limit yourself to only one type.
If you do keep multiple types, be aware that they are all the same species, and it’s likely they will breed outside of their type.
They are one of the most popular species for aquariums and are readily available at almost any fish supplier. Black mollies generally cost very little. They will usually cost you between $1.50 and $3.50 for each individual.
You will likely find both feeder fish stock and pet stock available. It may be tempting to reach for the feeders because their price is lower. Remember though that those fish are frequently not bred with the same quality and care as those intended as pets. They are, after all, designed to make it just to meal time.
For optimal results and the best stock, it’s always best to work with an experienced and reputable breeder. The little additional cost will be well worth it when you consider your healthy population won’t need to be replaced as frequently. You’ll also be lessening the likelihood of introducing illness to your tank.
Black molly fish are quite enchanting and have many fans—finding a good breeder to purchase from at a reasonable price should be easily possible. If you can’t find your source locally, you can always purchase them online and have them shipped to you.
When shopping for your fish in person, always make sure to choose from a tank with active fish. Lethargic or awkwardly moving individuals can indicate illness and should be avoided.
Can you Breed Them?
They are fun and easy to breed. Unlike most species, they are live bearing, which adds something to the experience.
For an optimal breeding experience, you’ll want many live plants and a lot of hiding places. Each time a black molly has young, you can expect 40 to 100 fry. Why the extreme number? These fry are very vulnerable to being eaten—even by their own parents.
The live plants in the aquarium will help provide the coverage your fry need to make it to adulthood. Of course, to entirely ensure their survival, your best results happen when you relocate the female to a breeding tank before birth, and remove her directly after. This will enable the fry to grow unaccosted.
If that’s not possible, or if great survival numbers aren’t as important to you, growing out your fry in the existing tank is possible.
Your black molly fry will eat the same foods the adults do. You’ll only need to account for their small size by making sure the provided food is small enough for them to eat. As they are tiny and, of course, have smaller stomachs, you’ll need to plan on feeding your new fry more often than usual—three or even four times a day.
Interesting Facts and Trivia
- They are closely related to guppies.
- They grow quickly and reproduce well—it makes them not only a popular aquarium fish but also a popular feeder fish.
- They have teeth and can actually chew their food.
- They used to belong to the family Mollinesia but are now considered Poecilia.
- Females can collect and retain sperm to save for later—for up to two months—in case anything should happen to the male.
- The larger the female, the smaller the fry—but the greater number of fry you can expect to have.
- Males will deliberately damage each other’s fins during fights to make their rival less desirable to the females.
- Mollies have been so heavily and selectively bred that finding fish like them in their original habitat is no longer likely.
- They have a jaw that’s able to protrude forward—this protruding jaw makes for an excellent tool when it comes to scraping rocks and wood for a quick snack of algae growth.
- Selective breeding for the traits we love has led to their smaller size—their wild cousins are much larger in size.
- Black molly are able to live in poorly oxygenated waters because of how they source their oxygen from the surface water—this is not the case for many other live-bearing species.
If you’ve been looking for a hardy fish that can thrive even in the care of a new hobbyist, the black molly is a great choice. They’re so fun and lovable, even the experienced fish keeper can enjoy having them in their tank.
It takes just a little bit of work to get your tank ready for black mollies. Don’t skimp on the size of your tank or the quality of your filtration system, and you’ll be off to a great start. Invest in hardy aquarium plants, even if they’re more expensive—they are notorious leaf munchers.
We hope you’ve learned everything you need to know here about black molly care. What’s your experience with them? Can’t imagine your tank without them? Think they create more mess than they’re worth? We’d love to hear your tips and opinions—leave a comment for us below.
Happy fish keeping!