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Pregnant Molly Fish 101: Behavior, Duration, Care & More

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A pregnant molly fish raises a lot of questions. That is particularly true if it’s your first time in this situation. This guide will take you step-by-step through the process of growing a pregnant molly fish. I will try to cover all the different aspects regarding this topic.

That includes identifying a pregnant molly fish and caring for it. I will also answer some common questions, such as how long are molly fish pregnant and whether you should separate the mother from other fish.

So, without further ado, let’s dive into it.

How Can You Tell If A Molly Fish Is Pregnant?

These signs indicate that a molly fish is pregnant:

  1. The molly fish’s belly starts swelling.
  2. The gravid spot will become darker.
  3. The fish will spend most of its time hiding.
  4. The appetite of a pregnant molly will decrease (and later will increase). 
  5. The fish will behave aggressively.

Bear in mind that not every swollen molly is pregnant. Male mollies may also become swollen, primarily due to an underlying disease. But if your fish is a female and presents the signs above, it is most likely pregnant.

Still curious? Click here for more information on how to tell if a molly fish is pregnant. I also discussed how to distinguish a pregnant molly fish from one that is fat due to an underlying condition.

Where Is The Gravid Spot On A Molly Fish?

You can find the gravid spot near the anal fin at the lower part of the molly fish’s abdomen. As it is actually the womb of the fish seen through the skin, the gravid spot will gradually expand as the pregnancy progresses. The spot will become darker and larger until the molly fish gives birth to living fry.

The gravid spot is one of the most recognizable hallmarks of a pregnant molly fish. However, bear in mind that it also appears when the molly fish isn’t pregnant. In some cases, it merely indicates that the fish in question is a female.

Click here to learn more about the location of the gravid spot on a molly fish. I also discussed whether you should be worried if you can’t find the gravid spot and embedded an excellent video to determine the gender of your molly fish.

How Does A Molly Fish Get Pregnant?

This is how molly fish get pregnant: 

  1. The male molly pushes the gonopodium into the female molly’s body. 
  2. The male transfers the milt, a substance containing his sperm. 
  3. The male’s sperm fertilizes the female’s eggs. 
  4. The fry will grow inside the female’s belly for 50 to 70 days. 
  5. The female molly eventually gives birth to living fry.

Unlike other species, female mollies carry unfertilized eggs in their abdomen. But they don’t lay these eggs to be fertilized by a male molly fish. Instead, the eggs are being fertilized while still inside the fish’s belly.

If this piqued your curiosity, click here to learn more about how molly fish get pregnant. This article includes a beautiful video that shows how a male and a female molly mate.

Can Molly Fish Get Pregnant Alone?

Molly fish cannot get pregnant alone. A female molly must be exposed to the male’s sperm to conceive. Yet, females can store the sperm in their ovaries for up to eight months. Therefore, the male doesn’t have to be in the tank once the eggs are fertilized.

Many aquarists notice signs of pregnancy without the presence of a male molly fish. But that doesn’t mean the female got pregnant on her own. It might have stored sperm from a different tank. If your fish is new to their tank, the sperm could be from the store.

Click here to learn more on whether molly fish can get pregnant alone. I made sure to list the most practical steps you should take once you realize that your molly is pregnant.

How Long Are Molly Fish Pregnant?

Molly fish are usually pregnant for 50 to 70 days. That period may shorten if the molly is provided with enough hiding places. Also, the gestation period of some molly species is shorter, such as sailfin mollies, which are pregnant for roughly 21 days.

If your molly fish is pregnant for more than 70 days, its chances to give birth are pretty low. However, it can be challenging to determine the exact duration of the pregnancy, as you can hardly tell the exact time the fish was conceived.

Do you feel unconfident? Click here to learn more about how long molly fish are pregnant. I made sure to list a few tips to ensure your molly fish delivers healthy fry in time.

How Can You Tell If A Molly Is About To Give Birth?

These signs indicate that a molly fish is about to give birth: 

  1. The gravid spot will take on a deep black color. 
  2. The molly’s belly will become square. 
  3. The molly fish won’t go out of its hiding spot. 
  4. The pregnant molly will show no interest in food. 
  5. The anal vent will turn larger than usual. 
  6. The molly will start shivering and breathing heavily. 
  7. The molly fish’s poop will become white.

If you notice these signs in your pregnant molly fish, it is better to leave it alone. Try not to move it to a separate tank or make drastic water changes at this point. As it is highly stressed, rearrangements may potentially kill your molly fish.

Still curious? Click here to learn more on how to tell if a molly is about to give birth. I made sure to discuss what steps you should take at this stage. Also, I included an excellent video that will help you prepare your aquarium for birth.

Do Mollies Eat Their Babies?

Molly fish will eat whatever fits in their mouths, including their babies. That habit becomes more noticeable when the fish is stressed due to inadequate water conditions, an overcrowded environment, and a lack of food.

As pregnant molly fish eat their fry, it is common to place a divider in your tank. This way, the babies will swim to the other side and escape their parents. If you don’t have a divider or the time to make one, introduce as many hiding spots as possible.

If that caught your attention, click here to learn more about how mollies eat their babies. I also answered whether mollies would eat their babies immediately and shared some practical tips to prevent this from happening.

When To Separate Pregnant Molly Fish?

It is best to separate pregnant molly fish from her tankmates when it starts showing signs of pregnancy. At this point, it is best to take away the companions while leaving the pregnant molly in the original tank. Then, the molly should be separated from the fry a few hours after it’s done giving birth.

A pregnant molly that shares a tank with other fish may fall victim to bullying and harassment. Bear in mind that pregnant mollies can hardly defend themselves, even though they become more aggressive during this period.

Feel unsure? Click here for more information on when to separate pregnant molly fish. Besides the actual timing, I also discussed the right way to separate a pregnant molly. That includes a video that shows how to make your own aquarium divider.

Do Mollies Have All Their Babies At Once?

Mollies have their babies in a dedicated period. During that time, baby mollies will pop out the birthing canal, one at a time. Eventually, the female molly will deliver between 40 and 100 living fry. Generally, the birthing period is not likely to proceed for more than a couple of days.

It is unlikely to find your molly fish making significant pauses during delivery. The fish may wait an hour or two from batch to batch, but not days. If the mother is still swollen after a couple of days, she might have aborted the fry.

Still curious? Click here to learn more about whether mollies have their babies at once. I made sure to discuss the particular cases in which mollies may fail to deliver their fry and how water conditions might affect that.

Why Is My Pregnant Molly Hiding?

Pregnant molly fish tend to hide to avoid danger. Because they are vulnerable at this stage, pregnant mollies choose to hide behind plants, leaves, rocks, etc. They may also be reacting to aggressive tankmates, including the male mollies themselves.

A pregnant molly fish that is hiding is merely taking care of herself. Getting behind plants and decorations allows her to grow the fry in peace and potentially give birth without harassment. It is perfectly normal, and you shouldn’t be worried about it.

Click here to learn more about why your pregnant molly fish is hiding. I also discussed what steps you should take if your molly fish is hiding and which to avoid.

How Do You Take Care Of A Pregnant Molly Fish?

Pregnant mollies may find it difficult to hide. They will manifest unmistakable signs, including swelling, loss of appetite, sluggishness, and a gravid spot. The care you give these mollies is vital. Just because a fish is pregnant doesn’t mean it will give birth successfully.

It can abort the babies or reabsorb them. Unlike a human, you don’t have to participate directly in the birth of molly fish fry. But you can use the following steps to create a conducive environment for a successful birth:

1. Separate Your Pregnant Molly In Time

Pregnant mollies spend a lot of time in hiding. Some of them will lash out at their tankmates, especially during the initial stages of the pregnancy. But as the pregnancy advances, they will hide because they don’t feel safe out in the open.

They don’t have the strength to fend off attackers. This is part of the reason why many aquarists keep pregnant mollies in separate tanks. They know that male molly fish are enthusiastic breeders, and they won’t hesitate to hound a female molly to death, even if that molly is pregnant.

A pregnant molly in a separate tank can give birth in a peaceful environment away from aggressive tankmates. But the timing of the transition matters. You don’t want to prolong the fish’s stay in the breeding tank because it won’t appreciate the isolation.

However, it is just as dangerous to move a fish on the verge of giving birth. The stress of the transition may cause lasting harm. If you want to move the fish, you should do so two to three weeks into the fish’s pregnancy.

If your home cannot accommodate an extra tank, add a breeding box to the main tank.[1] A breeding box allows the pregnant molly to stay in the community aquarium. However, it also protects the pregnant fish from male mollies and any other aggressive creatures in the water. 

A divider can work just as effectively. The objective is to separate the pregnant molly from the other fish, not only for the mother’s sake but to keep the babies safe once they are born. Choose any method that achieves this goal.

Once the mother gives birth, you have to separate her from the babies before she eats them. But she needs rest. You cannot take her back to the main tank. 

She doesn’t have the strength to deal with the harassment coming from the male fish. That is why I suggest keeping her in a different tank for a day or two before moving her back to the community aquarium.

If you don’t want to move the mother, here is a useful way to trap the fry once they are born:

2. Introduce A Few Hiding Spots

Plants are the last resort. You are better off creating a physical barrier that prevents other fish from interacting with the pregnant molly. If you have a particularly aggressive male, it will scour the tank until it finds the female. 

This is why many recommend using dividers and breeding boxes.[2] But if you can’t afford these tools, add more plants. Live plants are the best option because they add oxygen to the tank, but you can use their plastic counterparts.

Mollies will use whatever they can find to stay out of sight. That includes plants and decorations. The more hiding places they have, the more at ease they will be. The reverse is also true. Pregnant mollies in tanks that don’t have enough hiding places will struggle with stress. 

If you want to raise the babies in the community tank, they will appreciate the plants and decorations because they have to hide to prevent the adult fish from eating them. People think that fish fry are only at risk when you keep them in tanks with violent species such as cichlids. 

But that couldn’t be further from the truth. Fish are opportunistic eaters that consume whatever fits in their mouths. And unfortunately, molly fish fry are small enough to fit in the mouths of the most common aquarium species. 

3. Feed Your Molly Fish In Small Quantities

Pregnant livebearers will eventually lose their appetites. But that is not an excuse to starve them. They still require nutritional meals, so you don’t have to change their diet. They can eat flakes, pellets, brine shrimp, bloodworms, etc. 

The most important consideration is the quantity. Because these creatures have lost their appetites, you should give them several small meals throughout the day instead of two large meals.[3] Otherwise, you are going to saturate your tank with leftovers.

And if you don’t remove them, the leftovers will rot, producing ammonia, a substance that poisons fish. Try to sprinkle the food in places where the mollies can reach it.

Because the creatures are sluggish, they cannot fight for food. You cannot force them to eat. However, they are more likely to eat if you make the food easier to access.

4. Pick The Right Tankmates

Plants and decorations are not enough if you want the pregnant mollies to stay in the main tank. You have to remove all the violent fish. Look for species that can live peacefully with mollies, including danios, bettas, angelfish, gouramis, and platies, to mention but a few.[4]

Pay attention to the size. A 12-inch Oscar is more than capable of harassing and even killing a 3-inch molly. If that molly is pregnant, it stands even less of a chance. In case the size is not an issue, consider the temperament.

Smaller fish with violent mannerisms can become a threat to your pregnant molly because it is sluggish and vulnerable. By removing all the large and aggressive fish, you can maximize the molly’s chances of a successful birth.

5. Adjust The Water Conditions

You have to maintain the appropriate conditions in the aquarium. Poor quality water can harm a healthy molly. Pregnant fish are not healthy. They are stressed, exhausted, and lazy, so they are less active. If you expose them to terrible conditions, they may abort their babies.

To create a conducive environment for your pregnant mollies, start testing the water. Check the pH, temperature, and hardness. 

These are the ideal water parameters for pregnant molly fish:[5]

  • Temperature: 75°-80°F (24°-26.7°C)
  • pH: 7.5-8.5
  • Hardness: 10-25 dKH (178-450 ppm)
  • Ammonia & Nitrites: 0 ppm 
  • Nitrates: <30 ppm

You must test the water routinely to ensure that the parameters fall within these ranges. To measure the pH, nitrates, nitrites, and ammonia, I personally use the API Water Test Kit (link to Amazon). It is highly accurate and lasts for hundreds of measures.

6. Pick The Right Tank Size

Every species hates crowding, and mollies are no different. They can become violent or inactive if you force them to share a small space with large numbers of fish. Because pregnant mollies are already stressed, they won’t survive crowded conditions. 

You have to give them a minimum of 15 gallons. You need three extra gallons for each molly fish you add. If you can’t afford a bigger tank, remove some fish. You can give them to friends or sell them to your local fish store. 

7. Make Sure There Is Enough Oxygen

Pregnant mollies require more oxygen than non-pregnant ones, as the fry growing inside also breath. The last thing a pregnant fish needs is an oxygen deficiency. But that is what you will get if you have a crowded tank. 

That is why I recommend adding air stones or a pump. Usually, you can trust the filter to create enough agitation. But if you’ve detected low oxygen levels, air stones and pumps are acceptable solutions. I personally got the Hygger Aquarium Air Stone Kit (link to Amazon).

8. Maintain A Clean Environment

An aquarium filter can remove some of the pollutants, but it cannot keep the water clean unless you also carry out water changes. Do this every week. Keep the water changes small, no more than 15 to 20 percent.

Remember that the pregnant molly is already stressed. You don’t want to add to that stress by performing a significant water change. If the creature lives alone in a separate tank, reduce the size of the meals.

The pregnancy will rob the fish of its appetite; the molly fish may stop eating altogether. But you can’t stop adding food to the tank. You have to make sure the food is available just in case the fish decides to eat.

This is why it is so important to vacuum the substrate. Uneaten food will sink to the bottom, where it will rot unless you remove it. If you reduce the size of the meals, you will have fewer leftovers to remove.

Pro tip: If your molly is pregnant and will give birth soon, you’ll need to know a little more about the babies. On that matter, feel free to check my complete guide on molly fry.

Conclusions

If your molly fish is pregnant, it will likely give birth to healthy fry. That is how nature works. However, because your fish live in an aquarium, they are far from their natural habitat. That means you should make some adjustments.

The most important thing is maintaining a peaceful environment for the pregnant molly. Adding a few hiding spots and removing unwanted tankmates will make a huge difference. Besides that, keep growing your molly as it is used to. Try not to make drastic changes at this point.

References

  1. https://fishkeepingguide.net/molly-fish/separate-pregnant-molly-fish/
  2. https://aquariumtales.com/pregnant-molly-fish/
  3. https://small-pets.lovetoknow.com/pregnant-guppy-fish
  4. https://small-pets.lovetoknow.com/pet-fish-types-care/molly-fish-care-birth
  5. https://modestfish.com/molly-fish-care/