3 Signs When Molly Fish Is About to Give Birth (And What to Do?)

This entry is part 3 of 5 in the series Molly fish

As we talked about mollies in the previous post, we discussed the fact that mollies are fish that is easy to care for. That many beginners love to keep in their tank.

However, another thing to know is that Mollies are also easy to breed. After a few weeks of gestation, pregnant molly fish will spawn between 20 to 60 fries. Unfortunately, as the nature rule, molly fries are prey to other fish or even their parents, and not all of the fries will grow to adulthood.

So, we need to know:

  1. When pregnant molly fish is about to give birth and what we need to be prepare before this time
  2. How to tell if your molly is pregnant and how you can ensure that most of the fries will survive.

Here is everything that you need to know.

How to tell if a molly fish is pregnant

When you know how to tell if a molly fish is pregnant, you will actually see that it isn’t hard to spot a pregnant molly. There are some visible signs that she is pregnant if you know what to look for. If you want to breed with mollies, it is important to know and to remember these signs.

Physical signs to know if she is pregnant

These are the physical signs that she is pregnant. There are some behavior signs as well, but it is harder to recognize than the physical signs.

1 – Her belly is changing and gets squared off. Normal mollies have round bellies. But if she is pregnant, her belly will start to become more square and thicker.

2 – Black lines on her belly will be visible. This is a great sign if you have mollies that are light in color. Black spots close to the anal vent is also a proven sign that she is pregnant. These black spots are actually the babies you can see.

3 – She eats more than what she normally does. This is something that is a bit harder to spot if you don’t know how much they are eating in general. Not everyone is standing and looking at how much each fish is eating.

Signs that your molly is going to give birth?

Now that you know that your molly is pregnant, how do you know that she is ready to give birth?

This is something important to know because this is a vulnerable time for her and the fries. These are some behavioral signs that she might be ready to give birth.

Behavioral signs to know if she is ready to give birth

It might be time to isolate the molly when you see these 3 behavioral signs that she is ready to give birth.

  • She starts to swim less. Staying in one spot for a long time as usual.
  • She starts to hide behind plants or decorations in the tank. She even might stay close to the tank heater to stay warm.
  • During pregnancy, she eats more, but when she is ready to give birth, she will not have an appetite. And, she might start being aggressive towards other fish.

Should I separate pregnant Molly?

A question that many are asking is if you should separate pregnant mollies. The answer will depend on one thing. Do you want to keep as many of the fry alive, or don’t you care if they are getting eaten by other fish?

One thing to remember is that newborn fries are small enough for mollies and other fish to eat. They think that this is food, because of the size of the fry. Without enough hideouts, the fry will not survive in a large tank with lots of fish.

However, if you want to keep all the fries then you should follow these steps.

1- You should transfer the pregnant molly to a separate tank with water taken from the main tank before birth and put her back in the main tank after giving birth. She will eat her own babies.

  • The birth tank should have hideout places to hide from the larger fish or the mother.  These hideouts can be plants, decorations, or anything where they can hide behind.
  • The tank should use a filter. But you need to put a sponge, foam or something like it over the intake on the filter, because the fry can be sucked up in the filter.

>>Further Reading: If you’re looking for small filters for the birth tank consider checking out our reviews on:

1 – Best internal filter

2- Best HOB filter

  • You should also have a heater in the tank to keep the fry warm. The temperature for the tank should be between 72-78oF (22-26oC). This is their natural habitat’s water temperature.

>>Explore top rated mini heaters on Amazon

1 – Aqueon Aquatic Flat Heaters 5 Watts

(the most affordable heater for 5 gallon tank)


2 – Hydor Slim Heater

(for 2.5 – 10 gallon tank)

Some people are making use of a breeder net. This is where the fry is getting separated from other fish, but they are still in one aquarium. By this method, you don’t need to buy a new separate tank, filter and heater, but this might stress the mom out.

>>Explore Penn-Plax AN2 Aqua Nursery and Hatchery Aquarium on Amazon if you need a new separate tank.

2 – The fries should be in the separated tank for at least 2 weeks. After this period, you can place the fries in the main tank. The fries are now too large to be eaten by the other fish.

How to take care of pregnant molly

Pregnant mollies don’t usually need any special care during their pregnancy. This is one of the reasons why many people are breeding with mollies. But, there are a couple of things you can do to ensure that your pregnant molly is more relaxed. Remember that a stressful pregnant molly can abort the fries.

1 – The first thing that you need to do, is to provide the mollies with extra shelters or hideouts. They are always looking for some darker, quiet hiding spaces. Away from the other fish. The more hideouts you have in the tank, the better.

2 – You can also adjust the water to a warmer temperature. The pregnant mollies can feel more comfortable and relaxed if the water is warmer than normal. We know that the normal temperature for mollies is between 72-78 degrees Fahrenheit. So, just two or three degrees higher will be enough for the pregnant mollie to relax more.

>> Explore the most affordable thermometer heater on Amazon

Marina Thermometer

If this is hard to give these conditions to your pregnant mollies, you can always have a separate tank with all the dark spaces they need. And, you can then adjust the water temperature without stressing the other fish.

What is the gestation period of a molly fish? How many babies does Molly have?

The gestation period of a molly fish is around 60 days from being fertilized to giving birth. Something interesting that not many know are female mollies can store sperm for months and may fertilize eggs as often as every 30 days, even when no male is present in the tank. This might be why your female mollie is pregnant, even if you don’t have a male in your tank anymore.

 Mollies may release 10 to 60 live fish at a time. Depending on the condition of the female and the tank conditions.


Age for the male and female mollies can begin to breed

The age that the male can start to breed is at approximately 12 months.  Females can start to breed when they are 6 months old. 

What can mollies breed with?

Did you know that there is some other type of fish that mollies can breed with? These are the fish that mollies can breed with if you have these in your tank as well.

Mollies can breed with Guppies. This is because they are from the same species. Guppies and mollies belong mostly to the genus Poecilia, and this is making breeding with each other possible.

>> Related post: Guppy Molly Hybrid – Is It Possible?

Do Mollies give birth at night?

Yes, most of the time, mollies do give birth at night. This is to give the fries the best possible change in survival. However, there are some cases where mollies did give birth during the day.

This is why it is recommended to have a separate tank for pregnant mollies that are ready to give birth.


The moment that you know your mollies a bit better, you will easily see when your molly females are pregnant. This isn’t hard to spot. The best and easiest way to see that they are pregnant is the black spots in the anus area. These are actually the babies that you see. There are other signs that she is pregnant as well. The black line on her stomach and the fact that her stomach starts to become square.

If you want to breed with your mollies, there are a couple of things that you need to remember. The most important thing is that the fries need to be in a separate tank for 2 weeks. With this guide, you are getting everything you need to know about pregnant mollies, and them giving birth. Making it even easier to care for your mollies.

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