How Long Do Molly Fry Take To Grow & Reach Full Size?

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There is nothing more satisfying than watching your aquarium fish reproduce. However, each time it happens, new questions arrive. As my molly fish produced their fry, I kept asking myself how long it would take for the newborns to reach their full size. Over the years, I gained some experience with this topic.

It takes four to six months for molly fry to grow and reach their full size. At this stage, called fingerling, the fish have fully functioning scales and fins, and the females are mature enough to reproduce. At one year of age, the males are also sexually matured and prepared to produce offsprings.

As we move forward, I will teach you a few tricks to make your molly fish fry grow faster. That involves eliminating possible factors that might stunt their growth and compromise their health. Also, I will share step-by-step instructions on what to do once the fry have grown and are ready to meet the community tank.

Also Read: Molly Fry 101

How Long Does It Take For Molly Fry To Grow?

When molly fish fry are born, you have to separate them from their mothers. This is because adult mollies are just as likely to eat their offspring. However, you cannot keep them in a separate tank forever.

You have to reintroduce them to the community tank at some point, but only after they have undergone sufficient growth. But how long do molly fish fry take to grow up? The answer depends on what you mean by ‘growing up’:

1. Fry (Weeks 0-8)

Molly fish fry are relatively small when they are born. They are smaller than adult mollies’ mouths, which is why their parents tend to eat them. The objective of isolating them is to give the fry the time to grow to a size where adult mollies are less likely to confuse them for food.

That can take anywhere between one and two months. At this point, the fry is large enough to survive in the main tank without the other adult fish eating it. What distinguishes this stage is that the fry can feed on its own without being dependent on the yolk-sac.[1]

If the young fish are in a breeding box, you can move them to the main tank after two weeks. Breeding boxes are relatively small. You shouldn’t use them to house fish fry for more than fourteen days.[2]

2. Fingerling (Weeks 8-26)

Adult mollies have an average size of 4.5 inches.[3] Some mollies can reach 6 inches, but 4.5 is the average. It can take a molly fish three to four months to reach this size. It isn’t necessarily an adult at this point, but it has reached its full size.

By this time, the fish has fully developed scales and working fins. However, bear in mind that some mollies can take up to six months. A few molly fish types grow at an even slower rate. Black mollies are one example; it may take them ten months to reach their full size.[4]

3. Reproduction (Weeks 26-52)

Just because your fry has reached its full size at four months doesn’t mean it is ready to mate. Female mollies are ready to reproduce at six months. Nevertheless, with male mollies, you have to wait a year. So, you’ll have to wait 52 weeks before the new generation will be able to reproduce.

Also Read: When Do Molly Fry Get Their Color?

How To Make Your Molly Fry Grow Faster?

Follow these steps to make your molly fry grow faster:

  1. Eliminate stress by using a separate tank or a breeding box for the fry.
  2. Introduce plants to provide hiding places.
  3. Place your aquarium in a quiet place without human traffic.
  4. Keep the fry in a tank of at least 10 gallons.
  5. Mimic the day/night cycle by providing sufficient light.
  6. Set the temperature to 72-84 degrees F.
  7. Adjust the pH to 7.5-8.5 and the water hardness to 20-30dKH.
  8. Perform partial water changes once a week.
  9. Use baby brine shrimp, infusoria, and micro worms when feeding the fry.

Some mollies will never reach their full size because of factors outside your control, such as inbreeding and genetic defects. But if that doesn’t apply to your molly fish fry, their growth will only become stunted if you fail to provide conducive conditions in their tank. 

In other words, the best way to enhance growth in molly fish fry is to provide optimal conditions. That typically involves the following:

1. Creating A Peaceful Environment

Start by eliminating the threat posed by the parents. You have three options to consider:

  • Separate Tank – Place the pregnant molly in a separate breeding tank. Once it gives birth, take the mother back to the main tank. The fry can remain in the breeding tank until they are too large for the adult mollies in the main tank to eat.
  • Breeding Box – If you don’t want to separate your fish, add a breeding box. I personally use the Capetsma Fish Breeding Box (link to Amazon). The container sits inside the main tank, but it protects its inhabitants from the other fish in the aquatic environment. Place the pregnant molly in the breeding box. Once it gives birth, remove the mother and leave the fry behind. 

Many aquarists prefer the breeding box because they don’t have to worry about acclimating the pregnant molly fish to the conditions in a whole new tank, which would happen if you moved the mother to a separate tank.

  • Plants – Leave the fry in the main tank but give them places to hide from predators by adding plants. Don’t expect all your young mollies to survive. Adult mollies can still find them. However, you can save enough to raise the population of molly fish in your tank.

Of course, it isn’t enough to protect the young mollies from predators. You must also identify and eliminate the various sources of stress that tend to assault fish in a tank, including:

  • Noise – Avoid rooms with loud and sharp noises. They will disturb your fish, not just the young ones but their parents as well. Loud and sharp noises can complicate a female molly’s pregnancy.[5]
  • Traffic – You should also avoid locations with a lot of human traffic, especially if the people in question keep stopping to tap the tank walls.
  • Tank Size – Molly fish fry should be kept in tanks of at least 10 gallons. Like adult fish, they don’t appreciate overcrowding. The more fry you have, the bigger their tank should be.

Some aquarists think that the size of the tank affects the size of the fish. That is to say, molly fish fry will remain small if their tank is small, and they will grow to a large size if their tank is large. But that isn’t true. Fish have no way of controlling their size to fit the tank size. That being said, the stress of living in a small tank can stunt a fry’s growth.

  • Light – Fish fry require light. To be more specific, they require a regular schedule of light, that is to say, 12 hours of daytime and 12 hours of nighttime. Your lighting regiment doesn’t have to be perfectly partitioned. In other words, you don’t have to give the fish precisely 12 hours of daytime and nighttime. 

The objective is to stimulate the day/night cycle they would typically encounter in the wild. Avoid excessively bright lights. You are also discouraged from suddenly switching the lights on or off. The transition should be gradual.

2. Adjusting the Aquarium Conditions

Mollies cannot grow at a good pace unless the conditions in their tank are suitable. You have to keep the following in mind:

  • Parameters – Check the temperature and pH. Young mollies are expected to live in tanks with a temperature of 72 to 84 degrees F and a pH of 7.5-8.5. Also, the water hardness should be 20-30KH. Use a thermometer and testing kits to keep an eye on the parameters. Do not permit them to shift from the range your fry require.

To keep these parameters, this is the equipment that I use:

  1. To measure the pH, ammonia, nitrates, and nitrites, I use the API Aquarium Test Kit (link to Amazon). This bundle allows me to perform up to 800 measures, making it highly cost-effective.
  2. For the water hardness, I use the 6 in 1 Aquarium Test Strips (link to Amazon). That bundle also comes with a thermometer and a chlorine test kit.
  • Filtration – Add a foam filter to the tank. It will keep pollutants out of the water. There are better filters; however, foam filters are safe for fry.[6] A more powerful filtration system is just as likely to suck the fry in. It may also stress them by generating a stronger current than they would prefer.
  • Aeration – Young mollies are even more sensitive to oxygen deficiencies than their older counterparts. This is why you have to keep their tank aerated. A decent filter will ensure that the oxygen circulates evenly in the tank, but you can assist its operations by adding one or more air stones. In my tank, I use the Hygger Aquarium Air Stone (link to Amazon).

Plants can help you here as well. In the presence of light, they produce oxygen and consume carbon monoxide. But you should know that this process reverses in the dark, which is why you have to pay close attention to tanks in locations with prolonged darkness. The plants will lower the oxygen levels rather than raising them, especially if they are too many.

3. Performing Regular Maintenance

Is your tank correctly maintained? If it isn’t, the dirty conditions could stunt the growth of your fry. You have to do partial water changes once or twice a week. This will remove pollutants and lower the levels of ammonia, nitrates, and nitrites in the tank.

Where necessary, you should use conditioners to neutralize toxins, such as the API TAP WATER CONDITIONER (link to Amazon). It is also important to occasionally scrub the walls and objects in the tank. Rinse them thoroughly to prevent the residue from the detergent from poisoning your fish. A clean tank will encourage your molly fish fry to grow at the correct rate.

4. Feeding Your Fry Adequately

Some aquarists make the mistake of feeding their fry food rich in protein and nothing else. They do this because protein tends to enhance growth. However, this is a dangerous habit that rarely produces the results people expect because it creates a nutrient deficiency. In the long run, you will ruin the health of your young mollies. 

Molly fish fry require a balanced diet consisting of baby brine shrimp, brine shrimp eggs, and infusoria to vinegar eels and micro worms.[7] You should feed them four or five times a day. Their bodies require a lot of nutrients.

Also Read: What Do Molly Fry Need?

What To Do When Molly Fry Reach Their Full Size?

If your molly fry are large enough to survive independently, without their parents eating them, move them to the main tank. However, try not to throw them into the water. The transition should be gradual and done with caution. Otherwise, it may create tension between the fry and the aquarium fish.[8]

Follow these steps once your molly fry reach their full size:

  1. Install a breeding box in the main tank.
  2. Use a net to transfer the fry to the breeding box.
  3. Transfer only a few molly fish fry at a time without overcrowding the breeding trap. 
  4. Give the fish fry in the breeding box an hour to acclimate to their conditions before allowing them to enter the main tank.

Once the fish fry are in the tank, please take a moment to observe them. If they show signs of stress or if the other fish start antagonizing them, put them back in the breeding box and take the necessary steps to deal with the aggressive fish. 

If the fish fry have transitioned successfully, move the next batch to the breeding box. Repeat this process until all the fry have moved to the main tank. Don’t be afraid to move any struggling fish fry back to their breeding tank or box.


If your molly has just given birth to young fry, it means that they are already separated from the yolk-sac and are capable of eating on their own. At this point, it will take them up to 26 weeks to reach their full size. The females are also sexually matured at this point.

However, it will take six more months until the male fry can reproduce. You may make the newborns grow faster by keeping a peaceful environment and adjusting the water parameters. Regular maintenance is also critical for healthy fry.