Poecilia mexicana (Shortfin Molly Fish)

Poecilia mexicana

Last updated on August 15th, 2022 at 12:15 am

Poecilia mexicana, also known as Atlantic molly, short finned molly, or shortfin molly fish, is one of the most popular pet species in the aquarium hobby. They can be found in many pet stores, are relatively inexpensive to purchase, and are relatively easy to care for (given they are in the right water conditions).

These tropical fish originate from North America’s Gulf Coast region, where they live in brackish waters with temperatures of 68-72°F and salinity levels that reach 10-20 ppt. As long as you maintain their preferred water conditions in your aquarium, these hardy fish will be happy living with you!

Poecilia mexicana is an extremely common aquarium fish that offers many benefits to both beginning and experienced aquarists alike. For one, it’s one of the hardiest fish in captivity, with the ability to withstand harsh conditions and easily adapt to sudden changes.

They can be found in freshwater aquariums around the world, but few people understand the care requirements necessary to maintain them in captivity.

They are relatively easy to care for, which makes them an ideal choice for new hobbyists or families with children who want to add aquatic life to their home environment.

If you’re considering adding this fascinating fish to your own freshwater tank, read on to learn about its unique physical characteristics and how to keep it alive and healthy!

Origin and description

Poecilia mexicana

Poecilia mexicana, also known as the shortfin molly fish, originated in Mexico and can be found in many parts of the world such as South Africa, Australia, and New Zealand, just to name a few.

These fish are also native to Texas, Oklahoma, New Mexico, Colorado, Kansas, and Wyoming.

Many people keep this fish as pets in both saltwater and freshwater tanks. They are very popular because they are inexpensive to buy and have attractive colors making them fun to look at.

It is endemic to Mexico and northern Guatemala, where it inhabits freshwater marshes and spring-fed streams. This species grows up to 11 cm long and can be distinguished from other mollies by its stripe along each flank.

The IUCN has listed it as Data Deficient due to insufficient information on its population status and threats. However, habitat destruction due to urbanization may pose a significant threat in some areas. Short-finned mollies are popular aquarium fish because they are easy to breed and come in many different colors.

Golden Wonder Killifish (Aplocheilus lineatus)

Species profile

Poecilia mexicana

Poecilia mexicana is a species of fish in the family Poeciliidae of the order Cyprinodontiformes, native to Mexico and the United States (specifically Arizona and New Mexico). Despite their name, they are not related to mollies. They have long been bred in captivity as aquarium fish, but in recent years have become popular among aficionados of killifish as well.

It has an overall silvery color with black horizontal lines. They have a round, compressed body. The longest fish ever recorded was a poecilia mexicana of 11 cm (4.3 in). Males are generally smaller than females, with longer tails and shorter fins.

Females can grow up to 8 cm (3 in), while males can reach up to 5 cm (2 in). Shortfin mollies prefer temperatures between 18-25°C (64-77°F), pH of 6-8, hardness of 3-12 dH, and salinity of 1-30 ppt. Although they are primarily omnivores, they will eat small insects more if available. In captivity, their diet should include lettuce, spinach, cucumber, zucchini, and algae wafers for proper nutrients.

Poecilia mexicana habitat

Fish of this species live in freshwater and brackish water habitats in tropical regions. They are considered benthopelagic. They live in water with a pH between 7.0 and 7.5 and the temperatures are between 22 and 28 degrees Celsius. They don’t migrate. The Muddy River in Nevada, USA is home to these invasive species.

They can also be found in small streams and wetlands across Mexico. It’s worth noting that habitat destruction due to human activities have negatively impacted populations of short-fin mollies in some areas. As a result, these fish have been listed as near threatened by IUCN since 2004.

Poecilia mexicana size

The average length of Poecilia mexicana is 1.6 inches (4 cm) in length, however, they have been recorded to grow to a maximum length of 4.3 inches (11 cm).

Tank size

Due to their size and wide swimming capabilities, the minimum recommended tank size is 30 gallons (114 liters).

Tank requirements

Poecilia mexicana, also known as a Mexican molly, is a freshwater fish that can live in tanks with or without plants. If you choose to go with plants in your tank, ensure that you take care of them properly, if your plants die from low water quality or from being uprooted by aggressive fish, you’ll need to buy new ones.

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Also, make sure your fish don’t swim over your tank lights; they could get injured if they burn themselves on these bulbs. Other than that, it’s important to keep your water clean and well-filtered for optimal health. These fish are fairly resilient when it comes to temperature and pH levels.

They can survive in temperatures between 68-86 degrees Fahrenheit, but prefer temperatures between 75-82 degrees Fahrenheit. The pH level should be kept between 6.5 and 7.0 at all times, although they will survive outside of those ranges for short periods of time before dying off completely.

It’s best to test your water frequently so you know what sort of conditions your fish are living in. You may want to consider investing in an aquarium thermometer and a pH testing kit if you plan on keeping poecilia mexicana for long periods of time.

Poecilia mexicana tank mates

Poecilia mexicana can be kept in tanks with danios, tetras, and guppies. It should not be housed with other Poecilia mexicana due to its territorial nature.  Like most livebearers, they are best kept in small groups of 1 male to 3-4 females.

They will tolerate being kept with non-livebearing fish such as cichlids and angelfish but may become aggressive towards them if they feel threatened. Males will also display aggression towards each other during breeding season so it is important that only one male is added per tank unless there are multiple females present for them to breed with.


Poecilia mexicana

In order to successfully breed your short finned molly fish, you need to create an environment that mimics their natural habitat. Get a 30 gallon tank and fill it with gravel or sand. Next, get two compatible short finned molly fish and place them in your tank with plants, such as Amazon swords or Java Ferns for cover. The water temperature should be kept between 75-80 degrees Fahrenheit to replicate their natural climate.

Feed your short finned molly fish high-quality flake food once a day. If you want to increase your chances of breeding success, feed live foods like brine shrimp or daphnia once every other day. Your male will start displaying his colors more prominently when he is ready to mate with your female so look out for these signs.

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When they are ready, he will chase her around until she submits and then they will spawn. Spawning usually occurs at night and can last up to 2 hours. The eggs will float to the top of your aquarium where you can collect them with a net or siphon and transfer them into another container filled with clean water from your aquarium.

This container should have floating plants like duckweed on top so that oxygen can reach the eggs. You can also keep a constant flow of air bubbles going through it by using an air stone connected to an air pump or powerhead. The eggs hatch into fries after some days.

The fry will require brine shrimp nauplii or microworms for first foods. They grow rapidly and reach sexual maturity at about 3 months.

Are they aggressive or peaceful?

These fish are peaceful, but have a reputation for being nippy when they feel threatened. Some short-fin mollies have been known to attack and injure humans who come too close to their aquariums. Be careful when handling these fish; exercise caution and keep your hands away from their mouths at all times. When kept with compatible tank mates, short-fin mollies are unlikely to pose a threat to you or your other fish.

Poecilia Mexicana care

Poecilia mexicana

The shortfin molly fish or poecilia mexicana is an omnivorous species that can adapt to many conditions. They are fairly easy to care for, as long as you have a community tank with enough space and hiding places. They are generally peaceful towards other tank mates.

You should also be careful when adding new fish to your tank because they may become aggressive if there isn’t enough room in your aquarium for them all. This species does not tolerate poor water quality very well so it’s important to keep your water clean at all times.  Feeding your shortfin molly fish will vary depending on their age.

Newborn fries can eat small amounts of flake food while adults prefer pellets and frozen foods. It’s best to feed them twice a day but only give them what they can eat within 5 minutes otherwise uneaten food will pollute your water which could lead to illness.

Poecilia velifera (Giant Sailfin Molly)

Poecilia mexicana diet

The Shortfin molly fish is an omnivore. They eat a wide variety of live food, including freeze-dried bloodworms, brine shrimp, flake food, and algae wafers. However, they have a strong preference for frozen foods such as mysis shrimp or krill.

Some people even feed them pellets but they prefer a more natural diet.


In captivity, shortfin molly fish can live for 3-5 years with proper water parameters and care.

Parasites and diseases

Poecilia mexicana is also susceptible to several parasites, such as lice and nematodes. As a result, it’s often recommended that you treat your fish on a regular basis. You should also monitor their behavior to make sure that they are acting normally—anything from clamped fins to swimming erratically can be a sign of disease.

If your shortfin molly shows any signs of disease or illness, you should consult a veterinarian who specializes in tropical fish and ask them for professional advice. In some cases, shortfin mollies will die within 24 hours of showing symptoms of an illness. This means that if you suspect something might be wrong with one of your fish, you need to act quickly.


There are few predators of Poecilia mexicana. Although larger fish, like largemouth bass, sunfish, and perch, will eat short-fin mollies if they can catch them, these predators typically do not pose a threat to short-fin molly populations.

Do Poecilia mexicana make good pets?

Yes, short finned mollies make for great pets. They are very active fish and should be kept in a tank with plenty of room to swim and chase around other fish. Being omnivores, they are not picky eaters and will happily feed on flake food or frozen brine shrimp. Some breeders have reported breeding success with live blackworms, small feeder goldfish, or mealworms.