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How Many Fry Can A Molly Fish Have?

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It is pretty exciting to see a batch of newborn molly fry. But that immediately raises some questions. In my early days, I couldn’t help but wonder how many fry mollies typically have. That was crucial information as I needed to know what tank to get for them.

Molly fish can have 40 to 100 live fry in a single session. However, that isn’t guaranteed. Some mollies produce less fry because they haven’t fully matured yet or are highly stressed, primarily due to unstable pH and extreme temperatures.

As we move forward, I will elaborate on the factors that may force your molly fish to produce less fry. Then, I’ll show you how to maximize this number, including a list of tankmates to avoid.

A couple of days old molly fish fry, in its search for food.

Still curious? Feel free to check my complete guide on molly fry. There, I discussed how to care for molly fry, what they eat, how often to feed them, their growth stages, and much more.

How Many Fry Can A Molly Fish Have?

Molly fish are livebearers, meaning that their eggs are fertilized internally. The gestation period typically lasts 60 days, after which the molly fish will produce 40 to 100 live fry.[1]

Of the total number of fry, fifty percent are expected to survive. However, this doesn’t happen every time. The following factors will influence the number of molly fish fry:

1. The Female Molly’s Age

Younger mollies tend to produce less fry. Don’t be surprised if a first-time mother delivers five babies or less. Given time, the production rate will improve. The more reproductive experience a molly fish has, the more babies it will provide.

Some mollies will deliver fewer babies than expected after 60 days. But if they store sperm, they may produce more babies within the next 30 days.[2]

2. Environmental Factors

The conditions in the aquarium can negatively influence a molly’s reproductive habits. That includes extreme temperatures, fluctuating pH, crowded environments, diseases, and aggressive neighbors.

A study (Perceived Risk of Predation Affects Reproductive Life-History Traits in Gambusia Holbrooki) showed that predators could suppress reproduction in some fish species.[3]

They induced stress that reduced cluster sizes and increased the number of stillbirths. Therefore, the presence of aggressive fish can quickly lower the number of fry the mollies produce.

3. The Size Of Your Molly Fish

Surprisingly enough, the size matters. Studies have shown that larger fish produce more babies than their smaller counterparts.[4] This variable doesn’t sound particularly important until you realize that some mollies are larger than others.

Some mollies are just three inches long. But you also have varieties that exceed six inches. That difference in size can affect the number of babies.

4. Unexpected Abortions

As livebearers, mollies do not lay eggs. They produce live fish. Eggs in a molly fish tank signify an abortion or miscarriage.

In other words, you cannot expect every molly fish to give birth. Some of them will miscarry because of diseases, genetic anomalies, toxins, or poor tank conditions.

If you find eggs in your molly fish’s tank, you should take them out, as they won’t hatch. If you don’t, they will rot and contaminate your tank with ammonia.

A molly fish fry that has just been born, along with 40 more brothers and sisters.

How Often Do Molly Fish Have Fry?

Molly fish give birth every 60 days, but this doesn’t include the courtship period. Courtship can last several days, during which the male and female mollies will chase one another.

Their dance can look like violence, but the aggression is harmless. The same male fish can mate with multiple females during the same season. Conception occurs when he injects milt into the female molly.

With the eggs fertilized, the fry will develop over the next 60 days. At that point, you must separate the mother from her babies. Otherwise, the parents, as well as other fish, will eat the offspring.

It is worth noting that different molly fish have different gestation periods. For instance, black lyretail mollies give birth every 60 days.

On the other hand, a common black molly’s gestation period is just 30 days. White sailfin mollies are even more impressive. You can expect them to push out fry within 21 days.[5]

A pregnant balloon molly fish with a squared-shape belly.

How Do I Encourage Mollies To Produce More Fry?

The water conditions play a central role in the birth of molly fish fry. You can encourage mollies to produce more fry by implementing the following steps:

1. Maintaining A Stable Temperature

Don’t expose mollies to extreme temperatures. The water can’t be too hot or too cold. Aim for 75 to 80 degrees.[6] The wrong temperature will encourage the mollies to extend the gestation period because of the stress.

The temperature should also be stable. In fact, that is even more important than the range itself. Click here for more information on what equipment I used in my molly fry tank.

2. Preventing pH Fluctuations

Maintaining the appropriate pH (7.5 to 8.5) isn’t enough. Although, the mollies will thrive if you keep the pH within the acceptable range. However, the correct pH range won’t help if the pH keeps fluctuating.

Fish hate change, and a pregnant molly’s weakened state makes the creature more vulnerable to the destructive consequences of fluctuations in the pH.

Technically speaking, fluctuations are normal. In some cases, they are inevitable. However, you don’t want the changes to exceed 0.2 in 24 hours.

If the pH changes, you can use various tools to restore it, including water changes, increased aeration, products like baking soda, peat moss, etc.

But this will either raise or lower the pH. It won’t necessarily prevent the fluctuations. To do so, you should consider the hardness, as minerals will stabilize the pH.

Soft water doesn’t have a significant buffer effect. You can improve the water’s buffer capacity by adding calcium, magnesium, and other minerals.

You can monitor the pH by using a test kit. I personally use the API Water Test Kit (link to Amazon). This bundle lasts for about eight hundred measures, making it highly cost-effective.

To enrich the water with oxygen, I got the Hygger Aquarium Air Stone (link to Amazon). A sufficient amount of oxygen will allow nitrifying bacteria to thrive. That will prevent ammonia spikes and stabilize the pH.

3. Providing Enough Food

Does the tank have food? Are the fish eating? Some mollies extend their gestation period because they are starving.

Naturally, they don’t want to introduce their offspring to an environment that doesn’t have enough food. Others cannot eat because of stress and diseases. They will lie listlessly at the bottom.

As a rule of thumb, I suggest feeding your mollies the amount they can finish within two minutes. That will keep them satiated while keeping a clean environment.

The gravid spot in a pregnant molly fish that will soon give birth.

4. Keeping Bullies Away

You are more likely to record extended gestation periods in tanks with bullies. Fish don’t want to give birth in environments with predators. Don’t be so surprised if the pregnant molly reabsorbs the fry.

This happens in aquariums with stressful conditions. If the molly in question has stored sperm, it can conceive and give birth 30 to 60 days later, especially if the conditions improve.

I would personally avoid aggressive and predatory fish, such as:[7]

  • Tiger Barbs
  • Red Tail Sharks
  • Cichlids
  • Dwarf Pea Puffer
  • Severums
  • Rainbow Shark
  • Jack Dempsey Fish

5. Providing Enough Hiding Spots

As you probably know by now, stress plays a central role in the amount of fry molly fish have and the birth frequencies.

An excellent way to lower stress is to provide a sufficient amount of hiding places. That will allow the female molly to deliver in peace, making it less likely for her to reabsorb the fry.

I personally got the JIH Aquarium Fish Tank Decor Set (link to Amazon) for my fish tank. It comes with plastic plants and a cave rock, which are excellent for pregnant fish.

6. Taking Genetics Into Account

It can take some mollies 70 days or more to give birth because of genetic anomalies. This assumes that the mollies in question will give birth.

Some fish are infertile, meaning they cannot produce live fry no matter how often they mate with a male fish.

Do Molly Fish Give Birth All At Once?

It depends. Mollies cannot push all their babies out at the same time. They are too small to perform such a feat. The babies will emerge one at a time, tail first. They will straighten and swim away.

But mollies can force all their offspring out within the same session. Labor can last an hour, especially if you have a smaller batch of 20 or so fry.[8]

However, you must remember that mollies can produce a hundred or more offspring. And it isn’t always possible to push them all out within the same hour.

Some birthing sessions can take an entire day. It is rare for labor to exceed 24 hours, but it can happen, especially if complications occur.

The creatures typically give birth at night. Although, it is not unheard of for mollies to go into labor during the day.

Even though the newborns are smart enough to swim for cover, the parents can still find and eat the babies if you forget to remove them.

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Conclusions

A fully developed and healthy molly fish will produce between 40 and 100 fry in a single batch. But that won’t always happen.

The environment often won’t allow your molly fish to produce so many babies. That includes unstable temperatures, a fluctuating pH, and aggressive tankmates. 

First, make sure to remove predatory fish. Ideally, it is best to leave the pregnant molly fish alone in a dedicated breeding tank. Then, monitor the water parameters and ensure they fall within the desired range.

References

  1. https://small-pets.lovetoknow.com/pet-fish-types-care/molly-fish-care-birth
  2. https://lifeoffish.com/does-molly-fish-give-birth-all-at-once-surprising-answer/
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3923821/
  4. https://www.science.org/doi/full/10.1126/science.aao6868
  5. https://www.aquariumfocus.com/how-long-does-it-take-for-a-molly-to-give-birth/
  6. https://modestfish.com/molly-fish-care/
  7. https://www.aquariumsource.com/aggressive-freshwater-fish/
  8. https://www.petplace.com/article/fish/general/my-fish-gave-birth-now-what/