Molly Fish: Feeding, Breeding, and More

Molly fish are very popular with new aquarists for many good reasons! They are peaceful, easy to care for, and as easy to breed as Guppies and other livebearers!

Mollies also have some unique qualities of their own; they are formidable algae eaters and include some of the largest livebearers in the world! They can be a little more sensitive when it comes to water conditions, especially where salt is concerned.

But armed with the information in this guide, even beginners will find molly fish to be hardy and long-lived aquarium residents. So let’s dive into the exciting world of molly fish together!

What are Molly Fish?

Molly fish are some of the largest members of the family Poeciliidae! This group includes several popular aquarium fish, including the Guppy (Poecilia reticulata), Platy (Xiphophorus maculatus/variatus) and the Swordtail (Xiphophorus helleri).

All of these fish come from North and South America, where they tend to live in coastal waters where salt and freshwater conditions intermix. Like their relatives, molly fish love hard, alkaline water and need salt even more than the others, which are mostly freshwater dwellers.

Black Molly Fish Lifespan

Like many small to medium size livebearers the molly fish is not particularly long lived. 2 to 4 years is an average life, with 5 years being especially long for an older molly!

Types of Molly Fish

There are three main types of molly fish that you are likely to see in pet stores at any one time. That said, in the wild there are a few more that almost never make it into the hobby.

Common Molly

The common molly fish is often known as the sphenops molly in books. But in pet stores, you are most likely to run across the many common names for its different color morphs.

Some frequently seen colors include the Gold Dust Molly, Dalmation Molly, Black Molly Fish, and Orange Molly Fish. But things get a little complicated here because common mollies are so closely related to sailfin mollies that they have shared genes over the years. So you can often find molly fish colors shared between the different types of molly fish.

Fortunately, it is still easy to tell them apart because true common molly fish never have the extended dorsal fins of adult sailfin mollies. They are also quite a bit smaller, rarely growing larger than 2 inches long.

Common mollies also have no problem living in pure freshwater, something that sailfin mollies find difficult. However they can still be adapted to pure saltwater just like sailfin and yucatan mollies!

Scientific Name: Poecilia sphenops
Origin: Mexico
Length: 2 inches
Aquarium Size: 20 gallons
Ease of Care: Easy

Sailfin Molly

Sailfin mollies are almost as popular as common molly fish thanks to their impressive size and showy colors. The males also have huge, showy dorsal fins that they use to overwhelm rivals and charm mates with.

Sailfin mollies grow large; the females are massive and can reach up to 6 inches long. Therefore they need a larger aquarium once they are fully grown.

The sailfin molly is a little more sensitive to poor water conditions than common mollies are. But more importantly, they absolutely must have some salt in their aquarium water if they are to do well. The more you add, the better, but pure freshwater tends to be fatal to them over the long term.

Scientific Name: Poecilia latipinna
Origin: USA & Mexico
Length: 4 to 6 inches
Aquarium Size: 30+ gallons
Ease of Care: Intermediate

Yucatan Molly

The least common molly fish to find in pet stores is the yucatan molly. As its name suggests it is found mostly around the Yucatan Peninsula in southern Mexico. It looks almost identical to the Sailfin Molly, to which it is very closely related.

However the yucatan molly grows to be even larger. Males easily reach 5 to 6 inches while females can top 8 inches as adults!

It can be nearly impossible to tell the difference between sailfin and yucatan molly fish. If you aren’t entirely sure then count the dorsal fin extensions on your males. Yucatan mollies have up to 20 fin rays on the dorsal fin. Sailfin mollies, on the other hand, have 15 or fewer fin rays.

Scientific Name: Poecilia velifera
Origin: Yucatan Peninsula, Mexico
Length: 6 to 8 inches
Aquarium Size: 40+ gallons
Ease of Care: Intermediate

Balloon Molly Fish

The balloon molly fish is a little special because it is not its own species; instead, it is a variety of the common molly that has different genetics. Like a blood parrot cichlid, it is deformed at birth with a misshapen spine, head, and other organs that affects its ability to swim.

While some people find them cute, I always recommend against buying fish like these because balloon molly fish, blood parrot cichlids, and other deformed fish tend to have serious health issues. They also swim poorly, have weak immune systems, and typically die sooner than molly fish with normal bodies.

Molly Fish Care

Like all livebearers the molly fish is easy to care for and very hardy. They are also some of the easiest fish to breed, making them perfectly suited for beginners!

Molly Fish Aquarium Size

Aquarium size is fairly unimportant so long as you don’t go too small. 20 gallons is a popular size for beginners because it allows you to keep fish that are slightly larger, like mollies! Just remember that if you want sailfin or yucatan molly fish then you will need to move up to 30 or 40 gallons of space!

Water Parameters for Black Molly Fish

Water conditions for molly fish is one area where things can get a little tricky. Because mollies are the most salt-loving members of the livebearer family. Common mollies don’t require salt and get along just fine in freshwater.

Sailfin and yucatan mollies absolutely need to have some salt in order to do well. Without some aquarium salt they usually end up catching opportunistic diseases like ich or simply stop eating and waste away.

How much salt is subject to debate but I recommend a tablespoon per 5 gallons to begin with. This is enough salt to satisfy their needs without unduly stressing other fish in the tank that may be salt sensitive, like tetras.

Molly fish are also some of the best additions to a brackish water fish tank that you can find and will thrive in these conditions! Just remember that salt does not evaporate from an aquarium system once you add it.

So always top off evaporation with fresh water; don’t add salt to it because the salt that you originally added hasn’t left. Only add more aquarium salt to compensate for that removed during water changes.

Molly Fish Water Conditions

Other than salt, molly fish are extremely good choices because they prefer hard, alkaline water. This water chemistry is what normally comes from the tap in most countries anyway. So all you need to do is use a standard dechlorinator to remove chlorine and chloramine before using it for aquarium water changes!

Molly fish temperature should fall within a range of 70-78℉. Not too cold or too warm; remember these fish are even found in the coastal regions of the southern United States so they don’t need to be kept in extremely warm conditions.

Molly fish are also very resistant to high levels of ammonia, nitrite, and nitrates. Unless you prefer fishless cycling with ammonia, mollies are a great first fish to jump start the cycling process with!

12 Compatible Molly Fish Tank Mates

Molly fish are peaceful aquarium dwellers that can get along with a wide range of community tank residents! And here are a few of the best tank mates for Mollies:

Guppies

Care level: Beginner
Temperament: Moderately peaceful; varies by individual
Tank Requirements: Minimum of 10-15 gallons
Water Requirements: 74-82° F, 6.8-7.8 pH
Life Expectancy: 2 years

Guppies are very flashy fish, unique for their bright and varied coloration. Unlike many captive fish species, male guppies are smaller than their female counterparts.

Like mollies, they are small and quick, meaning they will get along well, enjoying friendly competition for food, but they should mostly stay out of each other’s way.

Guppies are similar to tetras and betta fish in appearance. Male guppies are more colorful, growing to a max length of 1.5”, whereas the females are grey and not as flashy but grow larger, about 2.5” in length.

Platys

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 13+ gallons
Water Requirements: 72-78° F, 7.0-8.3 pH
Life Expectancy: 3 years

Platies are small, fast, and colorful fish that are friendly and fun to watch. They have fan-shaped tail fins and come in a variety of colors, from black, blue, brown, red, gold, and green. There is even a cool variant of platy that has a unique black pattern on its tail that resembles Mickey Mouse’s head!

These freshwater fish are ideal for beginners and are best suited for 10 gallon tanks. Fortunately, they eat the same food as molly fish, making them even more ideal as tank mates. Platys grow up to be 1.5-2.5” in length. Like guppies and mollies, the females are larger than the males.

Danios

Care level: Beginner
Temperament: Generally peaceful but may nip fins
Tank Requirements: 5+ gallons per danio (best kept in groups of 5-6)
Water Requirements: 70-75° F, 7.0-7.8 pH
Life Expectancy: 2-3 years

Danios are a hardy schooling fish that comes in many varieties, one of the most well-known being Zebra danios (also known as zebrafish). Danios are slim little fish with forked tails, and most of the varieties have great color and horizontally striped patterning. They are very fast and prefer to be kept with a few other danios in a community tank.

Interestingly, danios mate for life! This means that if you purchase multiple danios, you might want to prepare for the possibility of baby danios very soon!

Tetras

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 10+ gallons
Water Requirements: 75-78° F, 5.0-7.5 pH
Life Expectancy: 8 years

Tetras are a slim little freshwater fish, best known for the neon varieties that add eye-popping color to the tank. They are social like danios and prefer to be kept in schools. They shouldn’t be kept with any fish with long fins as they are known to nip, so even though angelfish, gouramis, or other long finned fish could be good tank mates for mollies, they are not compatible with danios. Some species are more aggressive than others, and they need at least 20 gallon tanks.

Though tetras are omnivores in their natural habitats, they do well on flake food when kept in captivity. However, they would likely appreciate a snack of brine shrimp or bloodworms occasionally. For this reason, they might not be the best tank mates for shrimp.

Gouramis

Care level: Moderate
Temperament: Peaceful
Tank Requirements: 10+ gallons for dwarf species, 30+ for larger species
Water Requirements: 75-80° F, 6.8.7.8 pH
Life Expectancy: 4 years

Gouramis are really special and beloved by both beginning and advanced fish keepers. The dwarf varieties are easiest to find at most pet stores, and they are the most sociable.

Larger varieties may be more visually attractive with bright colors and interesting patterns, but they are also more aggressive and may not be as suitable as tank mates as their dwarf counterparts. All varieties of gouramis are hardy fish that will live upwards of 4 years.

Gouramis have an adaptation that makes them especially hardy. Their natural habitats in Asia are shallow waters with very little oxygen. For this reason, they have special organs called labyrinth organs that helps them breathe oxygen at the water’s surface. This adaptation helps them thrive in community aquariums.

Swordtails

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 15 gallons
Water Requirements: 70-82° F, 7.0-8.0 pH
Life Expectancy: 3-5 years

Swordtails are a freshwater fish species that are originally from central America, and in their natural habitat come in an olive green color. Being livebearers they are closely related to molly fish, guppies, and platies.

Swordtails can be found in a wider array of colors compared to wild fish, most commonly red or orange. They are very hardy pets, and are most identifiable by the long extension on the bottom half of their tail fin. Swordtails are peaceful, active community fish, making them great companions for mollies.

Angelfish

Care level: Beginner
Temperament: Generally peaceful, can be aggressive during eating
Tank Requirements: 30+ gallons per angelfish
Water Requirements: 78-84° F, 6.0-7.5 pH
Life Expectancy: 10 years

Angelfish are probably some of the most well-known hobby fish, popular due to their distinctive body shape and color as well as their graceful, hypnotizing movements.

Angelfish are native to tributaries of the Amazon River in South America. Their natural habitat includes shallow, slow moving water with vegetation and hanging trees that create shady hiding spots.

The biggest issue that hobbyists often face with angelfish is the tendency for smaller more aggressive fish to nip their fins. This means it is crucial to research the compatibility between angelfish and other fish in the aquarium to make sure that they are all compatible. Angelfish are peaceful, but can become territorial during breeding.

Cichlids (Some Species)

Care level: Moderate
Temperament: Known to be territorial and aggressive based on species
Tank Requirements: 20+ gallons
Water Requirements: 72-82° F, 7.8-8.5 pH
Life Expectancy: 10+ years

There are over 1,200 species of cichlids, so this is a very broad category to discuss. Cichlids come in all colors of the rainbow, from solid silver to blue-green spotted to red-mouthed!

Cichlids are native to the waters of Africa and Southern Asia, the majority of the species dwelling in large lakes. They are unique for their mating and breeding activities, which usually involve courting, nest creation, and protection of their young, unlike most other fish species.

Generally speaking, most cichlids species are friendly enough to be compatible with mollies, but to be sure it is important to check on the specific species. Some species that make good Molly fish tank mates are Dwarf Cichlids, Rams, Discus, Keyhole Cichlids, and Severums.

They need at least a 20-gallon tank for the smaller species and up to 55 gallons for the species that can grow 8 inches or more!

Cichlids may be vegetarian or omnivores in nature, depending on the species, but un captivity they can be fed a combination of flakes, brine shrimp, and cooked chopped spinach.

Endlers Livebearer

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 5+ gallons
Water Requirements: 78-80° F, 5.0-8.0 pH
Life Expectancy: 2-3 years

These fish are not as well-known as some of the others on this list, but they should be! Endlers fish are sometimes crossbred with guppies and are considered by some to be a species of guppy.

The males of this species feature a really unique coloration with black, red, and iridescent blue-green scales. They are a similar size and sociability level as mollies, making them ideal tank mates.

Endlers only grow to reach a length of about 1”, so they should not be kept with tank mates who might prey on them, such as Oscars. Endlers are omnivorous and need a diet that provides algae and plant matter as well as brine shrimp and bloodworms.

Minnows

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 30+ gallons
Water Requirements: 64-72° F, 6.0-8.0 pH
Life Expectancy: 3+ years

Though many people believe that the term “minnow” simply refers to any small feeder fish, this is actually not true. Minnows are their own type of fish, found in bodies of water all over the world.

One of the most interesting minnow varieties is the White Cloud Mountain Minnow, native to China. This minnow is silver and red, growing about an inch in length. Most other minnow varieties are a bit less flashy, brown or black with some grey or silver accents.

Minnows are very peaceful fish, but they are very small, meaning that for certain fish they may look like a tasty snack. They also require cooler water than some freshwater fish, so it is crucial to make sure that the water parameters align before introducing minnows to the tank.

Snails

ivory snail — By Chapulines – Own work, CC BY-SA 4.0

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 1-2+ gallons
Water Requirements: 75-86° F, 7.0-7.5 pH
Life Expectancy: 1 year

Snails are great additions to most fish tanks, as they will not bother the fish at all and they help keep the tank clean!

They feed on all of the waste of the tank, and are able to protect themselves from any fish who might be curious about them by simply tucking into their shells.

There are many aquarium snail species to choose from for freshwater tanks, including mystery snails, inca snails, apple snails, and rabbit snails!

Shrimp

Care level: Beginner
Temperament: Peaceful
Tank Requirements: 5-10+ gallons
Water Requirements: 65-80° F, 6.0-7.6 pH
Life Expectancy: 1-2 years

These tiny bottom feeders will not bother any freshwater fish they are kept with, including molly fish, and will feed on waste much like snails. They are fun to watch and will add an interesting element to the bottom level of the tank.

Freshwater shrimp can be very sensitive to changes in water parameters, which is important to keep in mind.

Different shrimp species have slightly different requirements for alkalinity and temperature, so be sure to read up on these requirements and make sure they fit with the fish you already have in the tank.

Red cherry shrimp, amano shrimp, ghost shrimp, and grass shrimp are all good choices that will keep the tank clean and can be very beneficial to the tank community- as long as they are not paired with any fish who might prey on them!

What Do Molly Fish Eat?

The best thing about choosing molly fish food is how unpicky they are! Molly fish are truly omnivorous and eat both plant and animal matter.

But a real key to success is recognizing that they are true omnivores, meaning they need both in order to be healthy. In fact, many aquarists don’t realize it but mollies are some of the best algae eating fish you can find in pet stores.

When hungry they will spend hours pecking at broad plant leaves, aquarium glass, and the gravel in your tank for green and hair algae. Not only do they love greens; they also won’t harm your live plants because molly fish don’t have teeth to take bites from tough plant leaves with.

Variety is the key to any molly fish food regimen! Supplement greens like algae with protein-rich prepared foods. A good flake food is best for them since the toothless molly fish won’t be able to easily eat a pellet that hasn’t softened in the water first.

I also recommend feeding them frozen food options like brine shrimp, blood worms, tubifex worms, and daphnia. Since any molly fish for sale comes from tank raised captive stock they are extremely unpicky and will eat almost as soon as you bring them to your new aquarium!

Molly Fish Breeding

Molly fish breeding is no harder than it is breeding guppies, platies, or any other livebearer. In fact, if you don’t see a pregnant molly in a few weeks of owning them, I’d be worried. So let’s first talk about telling the difference between male vs female molly fish!

Male vs Female Molly Fish

Sexing molly fish is not as easy as it is for guppies or swordtails but still not very difficult. One bright side is that many female mollies are as colorful as males, especially gold dust, dalmatian molly fish, and black molly fish.

The key here is to look at the anal fin! In all livebearing fish (and marine sharks) the male has a rolled up modified fin. In livebearers this is called the gonopodium and it serves the same function as the penis in male mammals: to internally inseminate a female.

If an adult molly has a gonopodium, it is a male. If the anal fin is normal and fan-shaped, it is a female; easy as that! Also, in sailfin molly fish, only the males have the hugely extended dorsal fin!

And behavior-wise, male molly fish tend to be more aggressive. They spend more time chasing both females and other males!

How Do Molly Fish Mate?

Mollies are livebearers so the male molly needs to insert his gonopodium into the female’s body cavity. Once he releases his sperm the female has the ability to hold onto it for an extended period.

This is why you might see a pregnant molly fish even if you only have a lone female; she was impregnated at the pet store but is only now developing fry months later! The mating affair itself is very brief so you likely won’t see the actual insemination occur.

But once it does the female will gradually begin to swell as her babies develop inside of her!

Caring for a Pregnant Molly Fish

Once you have a pregnant molly fish there is not a whole lot that you need to do. Simply provide her with warmth, good food, and clean water. Like most livebearing fish, mollies may also show a visible gravid spot as the pregnancy goes on!

The gravid spot is a dark patch that is the walls of the ovaries peeking through the thin skin of the pregnant molly fish. Since mollies have deeper colors than many other livebearers, you may not see this in your pregnant molly.

How Long are Molly Fish Pregnant For?

The female molly’s stomach will get much larger over the course of a few weeks. Usually it takes 21 to 28 days for the baby molly fish to be developed enough for her to give birth.

Do Molly Fish Eat their Babies?

Unfortunately, livebearers of all kinds are notorious for being willing to eat their babies in just a few hours after birth, and molly fish are no exception. Immediately during and after giving birth the female won’t try to eat her fry at first.

But eventually these maternal cues turn off because unlike us mammals, molly fish provide no care for their young whatsoever. The fry are born and then have to fend for themselves. In the wild, the baby molly fish would simply disappear into aquatic plants.

But in an aquarium the newborn molly fish are eventually discovered by their ever-hungry parents, who will gladly eat them like any other small prey. So you need to be proactive by either moving the pregnant molly fish to a breeding trap or by catching the baby molly fish shortly after birth.

Caring for Baby Molly Fish

If this is your first time having to raise baby fish then you are in luck! The reason why livebearers give birth to fry instead of molly eggs is because it allows the young to be born at a larger and more developed phase.

Molly fry are fairly large, as fish fry go. This makes it easy to feed them larger food, which is easier for aquarists to find. They love live prey to start, so try raising baby brine shrimp nauplii, if you can. Over time, you can transition them onto hand crushed flake food. Your baby molly fish will also find algae, infusoria, and other aquatic life to eat.