Poecilia mexicana (Shortfin Molly Fish)

Poecilia mexicana

Poecilia mexicana, otherwise known as the shortfin molly fish, is not your ordinary fish! This tropical freshwater fish hails from Mexico, and although it has never been popular in the aquarium industry, there are some notable differences between Poecilia mexicana and other brackish water fish that make it worth considering for those looking to add something new to their tank!

Molly fish, commonly referred to as mollies, are both brackish and freshwater fish from the Poecilia genus of the live-bearing family Poeciliidae. The poeciliids are some of the most diverse fishes in terms of morphology, behavior, and ecology.

Poecilia mexicana is one of the most popular and easiest brackish aquarium fish to care for. These fish are native to the U.S., specifically in Florida and Texas, where they thrive in temperatures between 68 to 86 degrees Fahrenheit with an ideal pH level of 6.5 to 7.5.

Origin and description

The shortfin molly fish is native to Mexico and Central America. Shortfin mollies are small saltwater fish that can be found in a wide variety of colors, sizes, and patterns. Mollies are often sold as starter fish for beginners because they are easy to take care of and breed readily.

They also have an omnivorous diet, which makes them ideal for people with busy schedules who do not want to spend time feeding their pets. It is important to note that some people may consider this species invasive because it has been introduced into many areas outside its natural range by irresponsible aquarists.

Species profile

Poecilia mexicana


The shortfin molly fish, or Poecilia mexicana, is a brackish fish in the family Poeciliidae. They are native to Mexico and Central America but have been introduced to many other parts of the world for use as aquarium pets. These fish are often bred in captivity and sold as inexpensive ornamental fish for home aquariums.

Marble Lyretail Molly (Poecilia sphenops)

Common name

The common name of the Poecilia mexicana is shortfin molly, short-finned molly, or Atlantic molly fish.


These shortfin molly fish live in both freshwater and brackish water habitats, usually near the edges of rivers, streams, and ponds. They are found in a variety of different climates, from subtropical to tropical.

Though they can be found in saltwater environments as well, they don’t do as well there as they do when living around fresh water. It’s possible that these fish have evolved adaptations that help them survive better when living around fresh water.

Poecilia mexicana size and weight

The shortfin molly fish is a brackish species of fish and measures at 10 to 12 cm (4 to 4.7 inches) in length. The weight of the shortfin molly fish is usually around 2 pounds, with males being slightly larger than females.

Poecilia mexicana tank size

A 30 gallons (114 liters) tank is the minimum size recommended for one Poecilia mexicana, but they do best when kept in groups of five or more, so a larger aquarium may be needd to keep them in groups.

Tank mates

Ideally, Poecilia mexicana should not be kept with other long-finned fish or other fin-nipping fish. In addition to Poecilias, other livebearers, and Armored catfish (Corydoras), Rainbowfish will also make good tankmates.


Poecilia mexicana

Similar to their cousins, Sailfin Molly females are larger than males, and they do not lay eggs. During breeding, a female produces between 10 and 140 live young. It is important to note, however, that not all of these young are born at once. Over the course of a year, she gives birth multiple times.

Poecilia mexicana needs a large birthing bin with plenty of vegetation in order to be separated from the general Molly population. She should be separated immediately from the newborns after giving birth to prevent them from becoming her dinner.

Bumblebee Goby (Brachygobius doriae)

Are they aggressive or peaceful?

Poecilia mexicana are peaceful fish that only fight with other mollies if there is not enough space for them to swim. They are typically active during the day and rest at night. Mollies like to live in tanks with rocks or plants to provide hiding spots so they can feel secure.

Shortfin molly fish care information

Poecilia mexicana

Shortfin molly fish are freshwater fish that originate from Central America. They are a schooling species, so it is best to keep them in groups of three or more. They can be kept in an aquarium with other peaceful community tank mates and will generally not bother other animals, as long as there is plenty of room for everyone.

The shortfin molly fish does best at temperatures around 72 degrees Fahrenheit but requires a heater if the temperature dips below 65 degrees Fahrenheit.

What they eat

Poecilia mexicana are omnivores and will eat any live, frozen, or freeze-dried food that you offer to them. They enjoy bloodworms, brine shrimp, flaked foods, and more. Give them some variety to make sure they’re getting all the nutrients they need.

Tank requirements

A 30-gallon tank is the minimum size for a shortfin molly fish. They are schooling fish, so they should be kept in groups of at least six. The tank should have live aquatic plants and should have a bare bottom or sand substrate. Shortfin mollies need a filter that supplies a strong water current.

The tank should be equipped with a heater and an air pump to provide them with oxygen, as they are surface breathers. Keep the temperature between 78°F and 82°F. Do not keep any other species of fish in their tank, because they will outcompete them for food. In addition, this type of fish needs quite a bit of space.

If you do not want your room to smell like algae, add some driftwood or rocks on the bottom which serve as natural filters.

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The tank should also include live aquatic plants and they require good lighting so they can see potential prey items around them. In order to prevent parasitic infections such as white spot disease, it’s important to maintain adequate levels of chlorine and nitrite in your water.


The lifespan of a Poecilia mexicana is about 3 to 5 years.

Parasites and diseases

Poecilia mexicana are susceptible to many parasites and diseases. One of the most common ones is Ichthyophthirius multifiliis, or Ich for short, a parasite that can be introduced to your tank by new arrivals from other tanks. It is easy to identify because it can make fish’s gills look like they have rice grains on them.

If you see this in your tank, you should quarantine any new additions for two weeks before adding them to the main population. Mollies are also susceptible to bacterial infections caused by Chilodonella, which will cause dark spots on their scales as well as fin rot.

Lastly, mollies can develop popeye if not fed enough protein. Popeye is an eye infection usually caused by a lack of iodine. Symptoms include bulging eyes with fluid buildup. Although often harmless, if left untreated, popeyes can eventually lead to blindness in one or both eyes.

Luckily this disease is easily treated with antibiotics and a few drops of artificial tears into each eye once per day until the symptoms disappear.

Predators (What animals prey on them)

Predators of the Poecilia mexicana include larger predatory fish, birds, and alligators. The shortfin molly fish is often preyed upon because it is slow and small. They are an easy target for larger animals. This behavior is also observed in other species of fish, such as the bluegill and white bass.

Do shortfin molly fish make good pets?

The Poecilia mexicana are often kept as pets because they are small, easy to care for, and don’t require a lot of space. This makes them an ideal pet for people living in small apartments or homes with limited outdoor space. They can also be used to monitor water quality and make sure that the aquarium has everything it needs for the fish.

Dalmatian Lyretail Molly (Poecilia latipinna)