How Does a Molly Fish Get Pregnant? The Ultimate Guide

Unlike other fish, mollies reproduce by delivering live fry. Fish that give birth to fully formed younglings are called livebearers. That said, it’s normal to wonder, “How does a molly fish get pregnant?” 

A molly fish gets pregnant when eggs are fertilized and hatched within the female belly. The fry begins growing inside, causing the abdomen to swell with time. Even though the growing fry are fewer, they usually develop fully so that they can fend for themselves straight off when they’re born.

In the rest of this article, I’ll discuss molly fish pregnancy and how to care for your fish to ensure a smooth pregnancy. You’ll also learn what to do when your fish is pregnant for successful reproduction and breeding. Keep reading!

How Does a Molly Fish Get Pregnant? Molly Fish Pregnancy Process

Molly fish’s pregnancy process takes about 45 days before bearing fry. The pregnancy comprises five stages, as discussed below:

1. Conception

Conception is the first stage of Molly’s pregnancy. During conception, a female molly receives sperm from the male counterpart to fertilize its eggs internally.

Interestingly, female mollies store enough sperm from a single mating to use later in the month for subsequent breeding. 

Mating will be fast, smooth, and guaranteed if you have both male and female mollies in the same tank.

Find Out: Why Do Mollies Die After Giving Birth? 8 Causes and Solutions 

2. Embryo Formation

How does a molly fish get pregnant

During this stage, the fertilized eggs develop into embryos. This process occurs seven days after fertilization

If you observe your molly fish carefully at this stage, you may see a gravid spot, as shown on the above image. This shows you that your fish is pregnant. 

However, the gravid spot is only translucent and darkens as the pregnancy advances to later stages. 

3. Embryo Growth Into Fry 

Embryos continue growing rapidly in size and features. Eventually, they transform into fry. At this point, your molly fish’s belly will begin swelling. 

The belly becomes more squared off and distended. Embryo formation takes the most time during pregnancy.

Also Read: How Many Mollies Should Be Kept Together? 6 Important Factors to Consider

4. Pre-Birthing 

The fry will take a few more weeks to develop fully and be ready for delivery. You can clearly see that your molly fish has a large belly. 

The fish’s gravid spot has also turned darker and more visible. Furthermore, you may see the fry’s eyes if you carefully observe the belly. 

5. Birth

Molly fish giving birth

After about 40-60 days, your molly fish will be due. Fry have developed fully and are ready for release into the water. 

So, the pregnant molly expels live fry. You should expect about 100 fries in one birth. Once they’re delivered, it’s recommended to remove the mother from the breeding tank to prevent her from eating the fry. 

Remember, the female molly can become pregnant again. As said earlier, the fish stores sperm from a single mating and retrieves it for future fertilization of her eggs. 

Molly Fish Pregnancy Symptoms

If you want to provide the best care for your pregnant molly fish and adequately prepare for the fry’s arrival, knowing pregnancy symptoms early is crucial.

Here are the most common molly fish pregnancy symptoms you should look out for. 

1. Swollen Belly 

This is the most common sign of pregnancy. Nonetheless, female mollies naturally have a more rounded belly that may confuse you about whether it’s pregnant. But you should confirm that the belly is more squared off and thicker—an obvious sign that your fish is carrying babies. 

As the fry continue developing, increasing in size, the abdomen also responds by expanding to accommodate them. That’s why you’ll see a more pronounced belly, especially in the later pregnancy stages. 

2. Isolation and Aggression

Pregnant molly fish will also change their behavior. Usually, they become lethargic, opting to isolate to quieter spaces in the aquarium. This is the primary reason you should have a separate breeding tank for the pregnant mollies. 

Isolation is essential to give your fish much-needed peace. They feel safe by hiding where there are few or no distractions. 

Notably, pregnant mollies may also behave aggressively towards other fish in the aquarium. 

Aggression is a defense mechanism to prevent potential threats that can otherwise endanger the babies being carried. 

3. Reduced Activity

Pregnancy takes a heavy toll on your fish, and the effects can be seen in decreased activity levels. Usually, fish appear tired and spend most time hiding and resting in secluded areas in the aquarium. 

The leading cause of reduced activity in pregnant mollies is the developing embryos. They use a lot of energy to grow. This is what drains mother molly, making her inactive. 

As the pregnancy advances, the mother molly becomes even more lethargic. 

4. Increased Appetite 

An increase in appetite is a common phenomenon in some pregnant mollies. As a keen aquarist, regularly feeding your fish high-quality food will be critical to their health. Remember, they’re carrying babies that need nourishment from the mothers. 

To compensate for the extra nutrition needs of the growing fry, pregnant mollies eat more food. While you provide food, ensure to provide a balanced diet with enough protein and vegetables

Nevertheless, don’t overfeed your fish. Overfeeding can lead to obesity, which puts your pregnant fish in potential health issues, including heart, liver, or kidney failure. 

5. Gravid Spot Darkening

Molly fish fry after birth


A gravid spot is an area near the fish’s anal fin. The gravid spot shows your fish’s reproductive status. 

When the molly fish is in advanced pregnancy, you’ll see a more prominent and darker gravid spot. The area darkens due to the growing babies in the belly, making it more easily distinguishable. 

You can use the darkened gravid area to estimate the pregnancy stage of your fish. If it’s only a bit visible, your fish has some time until she delivers. But if the spot is clearly visible and dark, it shows your fish is almost nearing birthing. 

Even so, some dark gravid spots may indicate a stressed or sick fish. Therefore, the darkening should be accompanied by other common pregnancy signs. 

6. A Liking for Warm Water 

The pregnant molly seeks warm aquarium areas since they’re tropical fish. Consequently, it’s crucial to maintain warm temperatures in the breeding tank to meet this need. 

The warm temperatures should run throughout the fish’s gestation period. Hence, the recommended temperature range for a pregnant molly fish is between 72 and 82℉ (22.22 and 27.78℃). 

Maintaining this temperature guarantees the fish a smooth run during pregnancy. Moreover, baby mollies will need extra heat to survive once released in the aquarium. 

7. Fry Eyes Become Visible

Later in the pregnancy, when the fry are nearing birth, you may see the fry’s eyes through the translucent skin of the mother’s abdomen. 

This means you should set everything up because it’s only a matter of time before the babies are introduced into the aquarium. 

How Do I Know If My Molly Fish Are Mating?

You can know if your molly fish are mating if the male is under the female and keeps bumping her belly. It’s actually a courtship process between the male and female. 

Here are the common signs that show mating in molly fish:

  • Chasing behavior: The male keeps chasing the female while nipping at her anal fin. 
  • Mating dance: You may see males perform unique mating dances that usually attract their female counterparts. The mating dance involves the male flaring their fins in a zigzag motion
  • Vibrant colors: During the breeding season, males usually develop more vibrant and striking colors to attract females for breeding. While females are less colorful, they also produce some vibrance to indicate they’re ready for copulation. 
  • The male uses the gonopodium fin to reach the female: Look out for the male and see if it’s trying to use the gonopodium to hook to the female. Gonopodium is a reproductive organ that helps the male attach to the female to deposit sperm. Usually, it looks like a modified anal fin.
  • The molly fish keeps hiding: Mating in mollies is so structured that the two mates engage in courtship behavior before mating. One of the ways to enhance the mating relationship is by the female hiding. If you see this, know your fish are mating. 

Notably, once the female gets pregnant, she will stop any further mating activities or behavior. However, the male may still want to continue mating. 

That’s why you may see females becoming aggressive towards male mollies since they know they should protect the babies they just conceived. 

How Old Does a Molly Fish Have to Be to Get Pregnant 

A molly fish has to be at least 3 to 6 months old to get pregnant.  Female mollies reach sexual maturity earlier than males (males take 12 months). That’s why in just six months, you have a molly that can have babies. 

That said, different mollies will have varying reproductive maturity ages because of factors such as:

  • The variety of the molly fish: There are many molly varieties. For example, the sailfin and common mollies typically reach sexual maturity between 3 and 5 months. On the other hand, the Dalmatian molly and the Black molly take about 3 to 6 months to reproductive maturity, whereas the Balloon molly takes 4 to 6 months. 
  • Growth rate: Fast-growing mollies reach sexual maturity earlier than slow-growing mollies. Growth rates are different depending on the breed and tank conditions.
  • Environmental conditions: If tank conditions are suitable, your molly fish will grow fast, reaching sexual maturity earlier. On the other hand, poor tank conditions, including excess impurities in the tank, will slow your fish’s growth and even kill them. 

What to Do If My Molly Fish Is Pregnant

If your molly fish is pregnant, transfer it to the breeding tank. There, you can provide the best-specialized care your pregnant fish needs. The most important thing is to give the fish a stress-free environment during the pregnancy period. 

In the breeding tank, your fish will enjoy the following benefits:

  • Less competition for food and space
  • No harassment from other fish, especially males that still need to mate. 
  • Close observation and specialized care ensure that pregnant fish are healthy and deliver safely. 
  • Once the fry are born, they won’t be at the risk of other fish. 

As the pregnant molly fish nears delivery, prepare a nursery tank. This is the place where you’ll transfer the fry after birth. If you don’t, the mother can eat its fries, leaving nothing for you. 

5 Challenges That Come With Molly Fish Pregnancy 

No doubt, molly fish pregnancy comes with its fair share of challenges. As a responsible molly parent, you should help your pregnant fish to overcome any obstacles during gestation. 

Here are five common challenges that come with molly fish pregnancy:

1. Pregnant Molly Is Not Dropping Fry

If you see this, you may get worried, but it’s vital to know that fish varieties have different pregnancy periods, although the acceptable range is 28-60 days. 

Your molly could be sick, for that matter. Ensure your veterinarian knows what’s happening for urgent help. 

2. Your Pregnant Molly Falls Sick 

When a pregnant molly falls sick, it’s often hard for her to carry the pregnancy until she is treated. 

Common molly fish diseases that may affect the fish during pregnancy include fin and tail rot, ich, velvet, and swollen gills.

Help your fish by ensuring early treatment of the disease. Always seek veterinarian support for accurate diagnosis and treatment. 

3. Overfeeding

You’ll notice that pregnant mollies have a high appetite. This is normal, and you should provide enough food. However, some fish tend to overfeed, leading to obesity.

When giving food, ensure you provide it in small quantities at a time. Don’t sprinkle plenty of food in the aquarium. 

A rule of thumb is to feed a pregnant molly two to three times a day, providing enough food to eat 10 minutes or less.

Final Thoughts

After demystifying the question, “How does a molly fish get pregnant?” you know what goes on from conception to birthing.

Female mollies have a unique reproductive system that stores sperm longer for later use. Therefore, they can get pregnant again after delivering fry.

A gravid spot closer to the fish’s anal fin is a sure to confirm pregnancy in your molly fish. The spot darkens as the fish nears birthing.

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