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How To Know If Molly Fish Is Pregnant?

Mollies are indeed a lovely addition to any aquarium. They are easy to take care of, but there are instances when your molly fish requires more attention than usual. One such instance is pregnancy.

Hence determining whether or not a molly fish is pregnant is something every aquarist should know about. So if you are wondering how to know if molly fish is pregnant, this article is for you.

We will talk about the signs to look out for to identify the pregnancy so that you can take to right steps to ensure the Molly babies are healthy.

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

How To Know If Molly Fish Is Pregnant?

how to know if molly fish is pregnant
To know if a molly fish is pregnant, you should look for some signs. These include a swollen belly, increased appetite, the presence of a gravid spot on the belly, sudden inclination to warm areas, and changes in behavior such as aggression, hiding, etc.

Mollies are livebearers; they give birth to live young (fry) and do not lay eggs. Hence it is recommended to separate the mollies from other fishes while they give birth to the fries.

Because, other fishes can eat the smaller molly fries, including the mollies themselves; hence you should remove the female molly fish from the tank after the birth of fires.

By paying attention to these signs, you will be able to determine if your molly fish is pregnant and take the appropriate measures.

Now, let’s look at these signs in detail:

1. Swollen Belly

One way to know about molly’s pregnancy is by observing if the abdomen is larger or rounder than usual. Since a Molly fish can carry up to 120 babies at once, her belly will become rounder as she gets closer to giving birth.

how to know if molly fish is pregnant

Although swollen belly or stretched belly is not always a reliable indicator, as female mollies usually have a prominent belly, therefore, it’s crucial to take into account other additional signs that are mentioned below to make a more certain judgment.

2. Behavioral Changes

There are some behavioral changes you should look out for to know about the pregnancy of a molly fish.

Mollies frequently show certain behavioral changes while pregnant. For example, a pregnant molly fish would swim very slowly due to the additional weight and size of the developing fry inside her body.

The mollies are also aggressive towards other fish during their gestation period, which is unusual for a peaceful fish. When bothered, however, molly fish won’t necessarily attack other fish; they will attempt to avoid any confrontation.

Moreover, pregnant mollies generally seek out areas of the tank where they can be alone, or you can say they hide more often during pregnancy so other fishes can not bother them.

3. A Gravid Spot Under Their Belly

Another way to tell if your Molly fish is pregnant is to look for a dark, triangular mark that appears towards the anal cavity near the anal fin. This mark that livebearers have is known as a gravid spot, a cluster of developing baby fry inside the female fish.

how to know if molly fish is pregnant

To ensure that your molly fish is not just fat after eating, you should always look for a gravid spot to be more certain.

The gravid spot will be more visible on lighter-colored fish and grows in size during pregnancy. As the eyes of the fires are the largest part of their body, upon closely looking at the gravid spot, you’ll also be able to clearly see the eyes of molly fries; for the best view, you can see them under a UV light in your fish tank. However, if you have a black Molly, it may be more difficult to see the gravid spot as it may be less visible on darker-colored fish.

4. Change in appetite

During the pregnancy, or you can say right after fertilization, it is very common for a molly fish to have a change in appetite. It is due to the increased metabolic demand and nutritional needs for developing embryos. Pregnant mollies usually have an increased appetite and will eat more than they normally do.

While feeding, you will notice the molly’s eagerness to eat as they tend to be more active and might even try to steal some of the food from other fish in the tank.

However, their appetite drastically decreases once they are near the laboring stage or are about to give birth to fries.

5. Mollies Will be Seeking Warmth

In addition to the above, once conceived, mollies show an interesting habit of swimming near the heating system in your tank since they need a heat source during their gestation period to keep them at a constant temperature.

Thus, warm areas are perfect for pregnant mollies to hang around.

Moreover, if you use an overhead UV light for your aquarium, then you will notice mollies hovering closer to the surface; that is because UV lights also emit a little bit of heat through the water.

One quick note – Once you notice the changes mentioned above, it is suggested to transfer the molly to a different tank so she can give birth to fries in a peaceful environment. However, make sure the water parameters of new tank are same as the main tank. Too much fluctuations in the water parameters can make the fish sick or even die.

If you are afraid to transfer a pregnant molly to a new tank, then you can install a breeding box, this is the safer option.

A Quick Glance At Stages of a Molly Pregnancy

A molly’s pregnancy can last anywhere from 35 to 45 days though it depends on the type of molly you have in your tank.

During the pregnancy period, you can notice the following stages:

Stage 1: Mating

Stage 2: Formation of the Embryo

Stage 3: Development of The Fry

Stage 4: Pre-birthing Stage

Stage 5: Birth of Molly Fries

How to Take Care of Pregnant Molly Fish?

how to know if molly fish is pregnant
Caring for a pregnant molly fish involves providing her with a warm and comfortable environment. As they rely on heat to regulate their body temperature, you should keep the temperature of your tank between 23 and 29 degrees Celsius. You can use an aquarium heater to ensure that the water is at the right temperature.

In addition to a warm environment, it is important to provide hiding spaces for your pregnant molly. It can be in the form of plants, caves, or other structures. Pregnant mollies may feel more vulnerable and stressed, so providing them with places to hide can help them feel more secure.

It is also important to separate the pregnant molly from the main tank. Doing so will prevent other fishes from bothering the molly and give the pregnant molly a safe and stress-free space to give birth.

Moreover, because the molly fish can eat its fry, you should use a breeding net to keep the fries safe.


So that’s it for today’s article. We hope now you are equipped with all the information to know if molly fish is pregnant.

In a nutshell, you should look for signs like swollen or stretched bellies, gravid spots, changes in behavior and appetite, etc., to determine that your molly fish is pregnant.

Once you are sure about the pregnancy, take good care of the fish to ensure the healthy birth of the fry.