How Does A Molly Fish Get Pregnant?

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I remember my first time seeing a pregnant molly fish. I was at a friend’s house, and my friend just happened to have one in his tank. The first question I asked him was, “how does a molly fish get pregnant?”. Over the years, after growing dozens of mollies, I gained some experience in this field.

The pregnancy process of molly fish involves the following:

  1. The male molly sticks the gonopodium into the female molly’s body.
  2. The male will transfer the milt, a substance containing the sperm.
  3. The female’s eggs will get fertilized by the male’s sperm.
  4. The fry will grow inside the female’s womb for 50 to 70 days.
  5. The female molly will give birth to living fry.

As we move forward, I will elaborate on the pregnancy process of a molly fish. Then, I’ll discuss what difficulties it may involve and how to deal with them. I will also share the ideal water conditions for pregnant molly fish, so you can ensure yours delivers healthy fry in time.

Also Read: Pregnant Molly Fish 101

How Does A Molly Fish Get Pregnant?

Mollies breed quickly and easily. In fact, on occasion, you have to stop them from reproducing. They can add as many as 100 fry to your tank. If you’re lucky, they will give birth every 60 days.[1] 

If you’re unfortunate, you may get 100 new fish every 30 days. The breeding cycle of a molly fish is not particularly complicated. Keep the following in mind:

Mollies are livebearers, which means that they give birth to live babies. But they can’t do this alone. A male fish has to fertilize the eggs in their bodies. This happens during mating.

The actual process of copulation is straightforward. The male fish will stick the gonopodium into the female molly’s body, transferring a substance called milt. The milt has the sperm that fertilizes the eggs.

The male molly can court its potential partner openly until the female permits the male fish to mate with it. The male molly fish can also sneak up on the female molly fish before aggressively mating with the creature.[2]

Mollies have a gestation of 50 to 70 days. During that period, the eggs will hatch into fry, and the fry will develop until the mother is ready to push them out. The fry will eventually pop out, one by one, during a specific period that can take a few hours.

Also Read: Can Molly Fish Get Pregnant Alone?

What Difficulties Accompany Mollies Pregnancy?

While copulation is straightforward, the mating process can create complications for various reasons:

1. Aggressive Behavior

Mating among mollies can look like aggression. The males will chase the females all over the aquarium. In some cases, the male fish will hound the female fish to death. 

Even if the two have already copulated and the female fish is pregnant, the male molly won’t hesitate to mate with the pregnant female as many times as it sees fit. 

Some female mollies will rebel for this very reason. They will prevent the male molly from mating with them, or the stress will cause them to abort the birth. This tends to confuse beginners.

They don’t understand why a female molly that has copulated on multiple occasions has failed to show signs of pregnancy. They don’t know that stress can prevent livebearers like mollies from conceiving.[3] This stress can also kill the female.

People solve this problem by creating a harem for the male fish. A male molly is happiest when it has multiple females in the tank. A large number of females will dilute its enthusiasm. 

Rather than breeding with one female numerous times, to the point where the female in question dies, it will mate with multiple females. 

This isn’t a good thing if you have a small tank. All those females will conceive and produce babies after a few weeks.

2. Fry Reabsorption

When the male fish maneuvers the gonopodium into the vent of the female fish, it inserts sperm that fertilizes the eggs. But the female has the option of storing that sperm in the fallopian tube.[4]

She can use some of it to fertilize her eggs while retaining the rest. This doesn’t mean she will give birth. In some situations, she will fall pregnant immediately and then reabsorb the fry back into her body as nutrients.

In others, the mother will fertilize multiple batches of eggs several months down the line. This can perturb newcomers who don’t understand how a female fish is making itself pregnant. 

3. Bad Eggs

Mating allows a male molly to fertilize the eggs in a female molly. But that doesn’t mean all the eggs will hatch into fry. Some eggs are not viable. They will maintain their round shape and yellow color because of factors like stress and low calcium levels.[5]

The female molly will either abort them or expel them along with the healthy fry when it gives birth. You can either discard these eggs or wait for the tank’s inhabitants to eat them.

Can Male Mollies Get Pregnant?

Gender matters because male mollies cannot get pregnant. Only the female mollies enjoy this privilege. Female mollies have dull colors, at least in comparison to their male counterparts.[6] If all your fish are equally vibrant, look for the gonopodium. 

This anal fin is more prominent in male mollies, and it looks like a tube. Female mollies have an anal fin as well, but it is triangular. Male mollies are more aggressive than females. They tend to chase the females around. You rarely see the reverse.

At What Age Can Mollies Get Pregnant?

Age is just as important as gender. Like humans, mollies cannot breed until they reach sexual maturity. The exact age that mollies must attain before they can reproduce will vary depending on gender.

It can take male fish 12 months to attain sexual maturity. On the other hand, females can start reproducing at half the age of the male fish. If your females are six months old, they can begin breeding.[7]

If you are not sure about the age of your mollies, try following their behavior. Molly fish that reach sexual maturity will start showing mating signs. You’ll see the males chasing the females, and in rare cases, vice versa.

Also, the gravid spot, located near the anal fin of the female molly, will become more prominent. The presence of a gravid spot doesn’t necessarily mean that the fish is pregnant. But it does mean that the fish is a female and ready to mate.

Can Aquarium Conditions Prevent Mollies Pregnancy?

The conditions of a breeding tank are vital. The wrong conditions can prevent mollies from mating. And if they’ve started copulating, unsuitable conditions may prevent the female fish from conceiving.

When mollies refuse to breed, aquarists quickly blame the issue on sterility among the fish. They may also blame the age. It doesn’t always occur to them to consider the conditions. 

Even if the female molly falls pregnant, it can refuse to give birth because the conditions in the tank are not right.

These are the ideal water conditions for pregnant molly fish:

  • Temperature: 77-80 degrees F (25-27 degrees C) 
  • pH: 6.7 and 8.5 
  • Hardness: 20-30 KH 
  • Ammonia & Nitrites: 0 ppm 
  • Nitrates: <20 ppm

To measure the nitrates, nitrites, ammonia, and pH, I personally use the API Water Test Kit (link to Amazon). Within five minutes, you’ll know if something is wrong with your water. This kit also lasts for about eight hundred measures, making it highly cost-effective.

I also suggest adding as many plants and decorations as the aquarium can accommodate. Some mollies require privacy to breed. Also, hiding spots will prevent your mollies from eating their babies.

Pay attention to the hardware, including the heater, air pumps, filters, and lighting. A malfunctioning heater can cause instabilities in the temperature. 

You have to worry about oxygen deficiencies with an ineffective filter because the filter will permit the water to stagnate, which is the last thing you want. To oxygenate my tank, I got the Hygger Aquarium Air Stone Kit (link to Amazon). It is incredibly efficient and quiet.

You have to create a conducive environment in the tank if you want your mollies to breed successfully. The work isn’t done simply because the female molly is pregnant. You can’t celebrate until the female gives birth to healthy fry.


Female molly fish get pregnant using their gonopodium and the sperm that is stored in their fallopian tubes. After the sperm fertilizes the eggs, your molly fish will remain pregnant for 50 to 70 days. Then, she will give birth to living fry.

To ensure this process takes place, you have to match your tank with the right temperature, pH, oxygen, and hardness. I also suggest that you introduce hiding places so that the parents don’t eat their babies. An aquarium divider will also work.

Also, make sure that you test the water for toxins, including nitrates, nitrites, and ammonia. All three should be kept around 0 ppm. If you identified higher levels, make sure you replace the water more frequently.