Molly fish are easygoing, peaceful, and easy to keep. But unlike other species that may lay eggs, the molly fish are livebearers. This means they hold their young, or fry, until birthing live fish. But how long does that take?
After an average of 30 to 40-day pregnancy, it will take a molly approximately a day to give birth to her fry. Many mollies can actually store sperms for months at a time, fertilizing their eggs themselves every 30 days. They can birth anywhere between 20 and 100 fry at a time.
In this article, we will explore the molly fish, discuss their average gestation cycle along ways you can tell if they are pregnant, and when they might be likely to give birth.
What Is a Molly Fish?
Due to how easy they are to care for, molly fish are quite commonly found in home aquariums. They are freshwater fish with 39 different varieties. As mentioned above, they are slightly unique in that they hold their young inside until giving birth to live fry.
Some common species of molly fish include:
- Common black molly – short, 30-day gestation period
- Dalmation molly – long 60-day gestation period
- Black sailfin molly – about 40-day gestation period
- White sailfin molly – as little as a 21-day gestation period
- Black lyretail molly– a 60-day gestation period
When breeding your molly fish or trying to ascertain when it might give birth, always research the specific breed for average gestation periods.
Provided they are in a well-kept and peaceful aquarium, you can expect your molly to live as long as five years. They are both hardy and easy to care for, eating mainly plants and algae.
When looking to bring mollies into your aquarium, it is a good idea to have a group of four, as they like to school together. If you are not planning on breeding them, do not include a male in the group, as they are often the cause of aggressive behavior.
For four mollies, you will need at least a 10 gallon (37.85 liters) aquarium. Each subsequent molly fish will require a further 3 gallons (11.36 liters) in order to be comfortable.
How to Tell If Your Molly Is Pregnant?
It is not uncommon to find that your new molly fish is already pregnant before you introduce her to your aquarium. Given that they can store sperm and fertilize their own eggs, they are often gestating even when not kept full-time with male fish.
Some key things to look for if you think your molly fish might be pregnant include:
- You may see a change in behavior, such as slower movements
- Some mollies will seek hiding spots, such as in taller weeds
- In some cases, the molly may become more aggressive to their tankmates
- On lighter colored molly fish, you may see a dark triangular spot near the anal vent, called the gravid spot
- This spot will become bigger and the birth date approaches
- You should notice a fuller figure in your mollies as their stomachs stretch out
- In some very lightly colored mollies, you can even see the eyes on their fry through their translucent stretched skin
- A black line will appear on their bellies, though it will not be visible on darker fish
Generally, you should see a couple of days of courtship between the male and female mollies. You might see them chasing each other around the tank, and in some cases, there may be some signs of aggression between the two.
Once the male has deposited its sperm, the female will fertilize the egg. This begins an average 30 day gestation period. However, it has been known to be slightly shorter or longer. For example, the Dalmation molly is pregnant for 50-70 days and gives birth to grown young fish that do not need to be kept separate.
During this 30 day period, it is normal to see your molly fish start to slow down. Although in some situations, this will only be just before birth.
Signs Your Molly Is Close to Giving Birth
Regardless of their movements in the weeks leading up to the birth, your mollies will significantly slow down in the days before giving birth. At this point, you should be able to visually inspect the gravid spot. It will be quite dark and much larger.
The belly should be rounded, full, and in lighter species, quite translucent. At this point, the eyes on their fry should be visible.
It is at this point that you might begin to see aggressive behavior from your mollies. Though they will generally seek out a hiding spot, you may find that they become aggressive should another fish come near their hiding spot.
At this point, if possible, you should try to separate your molly fish from others in the tank. This will prevent the aggressive behavior and ensure that the pregnant, slow-moving molly can still feed.
In the video below, you can see how to set up a breeder box, including plants:
The idea behind the breeder nets is to protect the fry, not only from other tankmates but from the mother molly. Like the other fish, she will likely eat the young herself. If you can keep an eye on the tank, watch for signs she is close to birth. You don’t want to put her in the net too soon, but rather just before.
After a day, when you are sure she has given birth to all her fry, remove the mother back into the main tank. Leave the fry in the breeder net to grow in peace.
How to Care for the Fry?
If you are planning on breeding your molly fish, you should invest in a breeding tank. This is used to keep the pregnant fish separate and is a safe space for the fry to grow. This can be a separate five to 10 gallon (18.93 to 37.85 liters) tank that is set up much like your main tank. It should include rocks, a filter, and plenty of plant life.
To ensure survival once the grown fry are introduced to your larger tank, use the same water in your smaller tank to avoid any shock from changes in pH and temperature.
However, you could also purchase a breeding box that hangs in your aquarium. They often have small holes in the bottom through which the fry can escape, separating them from their mother. It is advised to add plant life to this box also, to make it more comfortable for the mother molly.
The fry should be kept separate from the rest of the tank until they are large enough to not be considered prey. They should be fed top-quality fish flakes or baby brine shrimp. They should be fed in small quantities multiple times per day.
It can take between four and six weeks until the fry are large enough to go into the main tank. Although always be sure that the young are bigger than the mouth of the biggest fish in your aquarium.
From the start of the breeding cycle, when the male fertilizes the egg, the average female molly will be pregnant for approximately 30 to 40 days. During this time she will become slower, seek out a safe place to hide, and you will notice growth in her belly.
Once she begins to birth her fry, it can take up to a day for all the young to be birthed. At this point, you should try to remove the young from the main tank, for their safety.
It is not uncommon for mollies to fertilize their own eggs, using stored sperm. This can happen as often as once a month and they can birth up to 100 fry at a time.