If you are sitting on the fence about commencing your journey in fish breeding, become a molly breeder. They are the easiest fish to breed and can provide a good learning experience.

This article aims to cover everything you need to know about breeding molly fish, identifying pregnant molly fish, and caring for them pre/post pregnancy, including caring for the molly fry(s).

If you are caught unawares and find newborn molly frys swimming in the fish tank, continue reading about the next steps and your options.

Note:  The information herewith is generic and applies to all types of molly fish like dalmatian, black, balloon, short-fin, sailfin, lyre tail, etc.

Basics of Breeding Molly Fish

The ideal ratio of male and female mollies in the aquarium is one male for three females to reduce aggression and stress while breeding.

If you do not plan to breed them, add only single-gender fish.

Female mollies preserve sperm from earlier mating in their bodies and use it to fertilize new eggs after 30 days without a male fish. Hence, add juvenile fish only to avoid any surprise(s).

Different types of mollies may breed with each other (interbreed), and mollies may cross-breed with guppies in the same tank, resulting in muppies or gollies! Add accordingly.

Sexual Dimorphism

The following image and table explain the differences between male and female molly fish.

differences between male and female molly fish
Attribute Male Molly Fish Female Molly Fish
Anal Fin Shape (On Lower Body) It is straight, horizontal, and pointed (as it has a gonopodium). It has a standard shape like other fins.  
Body Size and Shape Between 2.5 – 3.5 inches. Slimmer. Up to 5.0 inches. Rounder abdomen.
Dorsal Fin (On Upper Body) Relatively larger and longer in size than females. Comparatively smaller than males.
Body Colors Attractive, brighter/more vibrant colors. Relatively dull colors.

How to Breed Mollies

Once mature, mollies reproduce on their own. Male mollies breed after they are a year old, and females after reaching the age of 6 months.

The females reproduce every 30 days for the next 2.0 – 2.5 years during their life span of 3 – 4 years.

Experts warn against mating very old female mollies, as it impacts their health and the quality of the fry(s) they produce.

Molly fish do not lay eggs but give birth to very tiny live molly frys that swim freely in the water from the moment they are born.

Mollies give birth to 20 – 100 fry(s) every time, depending on their age and type. Few hobbyists have reported 120 fry(s) in a single lot. The number of newborn fry(s) increases with every subsequent pregnancy.

Mollies lack parental instincts. Hence, it is vital to separate the newborns from their mother/adult tank mates at the earliest.

Some allow mollies to reproduce in the main aquarium, with sufficient hiding spaces. Many fry(s) may fall prey to adult fishes or starve to death if the food meant for them does not reach them/they fail to eat it. It means survival of the fittest and controls the molly population. Though inhuman, this is how they survive in the wild.

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Alternatively, some use a separate breeding/spawning tank to protect all fry(s). It also helps feed fry(s) separately, improving survival rates.

Preparing Breeding Tank

A matured breeding tank should be ready before mollies breed. Set up a make-shift container/bucket using water from the main aquarium, if it is sudden, while you arrange for a separate tank at the earliest.

A 20 – 25 gallons tank is preferred. It should not be too large to exhaust the fry(s) while swimming to feed and provide sufficient free space to swim/grow.

Add thick live plants, rocks, caves, etc., for them to hide. Use a sponge filter to avoid fry(s) from getting sucked and dying. Subdued lighting with a day/night cycle, an air stone, and an air pump complete the spawning tank. Allow the tank to mature and break its nitrogen cycle.

Arrange a breeding tray that can fit inside the breeding tank. Tiny openings in the bottom of the tray allow the fry(s) to swim down to safety immediately after birth, keeping the mother (in the breeding tray) on top.

Start culturing infusoria and green water when you separate a breeding pair from the main tank to save on expensive fish fry foods from the market. Many breeders set up a brine shrimp hatchery too.

The ideal temperature for breeding molly fish is between 79 – 81° F. Maintain a pH of 8 (or 7.5 – 8.5) and water hardness of 15 to 30 dGH.

How do Mollies Mate

Acclimate and move the breeding/matured pair(s) to the spawning tank. Maintain quality aquarium water and feed protein-rich food to promote breeding. Perform 10-15% partial water changes weekly.

Mating behavior:

  • Male molly will chase female molly.
  • It will become territorial about female(s) if there is more than one male fish in the same tank.
  • Female mollies are choosy. They prefer brighter and larger males to produce quality offspring.
  • Female fish may swim away/hide from the chasing male if it is unwilling to pair or stressed.
  • Add more female mollies to raise the male/female ratio to 1:2 or 1:3.
  • After several failed attempts to persuade the same female, he will chase a different female molly till he gets a favorable response.
  • If a female molly is ready to pair, she turns sideways. The male goes underneath/atop the female during the act, ejecting sperm into her abdomen (using gonopodium) to fertilize her eggs internally.
  • A male molly mates with multiple females while breeding, if available.

Identifying Pregnant Molly Fish

Below are the pregnancy indicators:

  • Swollen/bloated belly despite usual feeding and good health. It becomes squarish in the final pregnancy stage, as depicted in the below image.
Identifying Pregnant Molly Fish
  • A dark spot above the anal fin on the abdomen, known as a gravid spot, is visible on the molly. It is prominent in light-colored fish but not easy to spot on a pregnant black, panda, or dalmatian molly fish, due to the dark colors/patterns on their bodies.
Pregnant Molly Fish Gravid Spot
  • An increased appetite. Becomes extra active during feeding time to grab more food to meet higher energy needs.
  • Becomes aggressive and prefers to keep aloof from other tank dwellers to protect herself.
  • Preference for warmer sections in the aquarium (near the heater vent/UV light) for extra warmth.
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Pregnancy Cycle of Molly Fish

The gestation period of most molly fish varies between 45 – 60 days, depending on their type. Dalmatian mollies have the longest gestation time of 70 days.

During the gestation period, pregnancy advances through various stages till the fry(s) are born. The broad pregnancy stages explained below are similar for all variants.

Pregnancy Cycle/Stage of Molly Fish

Pregnant Molly Fish Care

Isolate the pregnant molly fish at the earliest to help her relax. Use a heater to keep the water warm if need be.

If you used a breeding tank, remove the male molly fish from the spawning tank once the mating is complete. Or, the female molly fish should have enough hiding spaces in the main tank to avoid incessant male chasing. She may abort/deliver stillborn fry(s) if stressed.

Continue feeding her with protein-rich food and maintain the water quality. Take all possible actions to keep the mother fish stress-free.

Behavior of Molly Fish About to Give Birth

As explained in the diagram in section 2.0, the mother’s swollen belly will extend to her gills due to fully developed fry(s) occupying most of her body space internally.

The gravid spot will be at its darkest. Tiny, black eyes of the unborn fry(s) will be visible through the mother fish’s stretched translucent belly skin.

The pregnant molly fish will become more sluggish and relax most of the time in the tank by hiding behind decor/plants in dim light.

Her appetite will decrease as the delivery time nears. She will wriggle her tail more frequently to relieve the pain while delivering fry(s).

All these are indications that she will deliver at any moment.

Molly Fry(s) delivery and Care

Attach a breeding tray to the breeding tank. Move the mother molly fish (ready to deliver anytime) inside the breeding tray. Once she completes delivering all the fry(s), allow her to relax for a while before moving her to the main tank. It may take a few hours or a day.

If delivering in the main aquarium, ensure she gets her space without interference from other tank dwellers.

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Move her into a container filled with aquarium water to deliver if needed. Alternatively, move the most interfering fish(es) away.

Watch a pregnant molly fish deliver babies.

Infusoria, green water, and freshly hatched brine shrimp are the best fish fry foods in the first two weeks. Feed them 4 to 5 times daily in small portions that can fit into their tiny mouths. Be patient and direct them to the food, as they might take time to recognize that it is edible.

Remove any dead fry(s) immediately from the tank. Perform 10% partial water changes every day. Fry(s) need pristine water quality to survive and grow.

After two weeks, feed them the same food as adult mollies (in smaller portions) after finely powdering/crushing it. As they grow in size, reduce the frequency of feeding and increase the feeding portions according to the size of their mouths.

Shift them to a grow-out tank if the breeding tank falls short. Once they are large enough to NOT fit into adult fishes’ mouths, move them to the main aquarium.

Molly Fry(s) delivery and Care

Frequently Asked Questions (FAQs)

My molly fish is pregnant. Is it true that mollies deliver babies at night only?

Pregnant molly fish prefer dim light and hiding spots to feel safe during pregnancy. Many hobbyists have reported that their mollies gave live birth at night. However, it is not always true.

Mollies can give birth to fry(s) anytime during the day/night, when they feel heavily loaded, and have no option except to push out the fry(s). They seek darker spots in hiding to deliver babies, for safety.

How to prevent mollies from breeding?

As discussed earlier, adding single-gender molly fish and buying juvenile female mollies to avoid any surprise pregnancy can prevent mollies from reproducing. However, they may still interbreed or cross-breed in community tanks. Refer to section 1.0 for details.

Many experts lower the aquarium temperature to the lowest their fishes can bear. It reduces the propensity to breed.

Alternatively, many use the survival of the fittest approach to control the molly population in community tanks. Refer to section 1.1 for more details.

Parting Thoughts

Now that you are ready to breed mollies, do it responsibly.

Check with your local fish stores if you have too many mollies to handle. Many buy healthy fry(s) for cash, discounted price, or provide store credit/discounts in return. You can use it to purchase anything else from them.

Alternatively, connect with hobbyists for any takers. Exchange your excess mollies for another fish of your choice with them. If nothing works out, all local fish stores and aquariums accept fish-back donations. Leave your fry(s) with them.

Another challenge in mollies is inbreeding, which eventually impacts their quality and may cause diseases due to close genetic mutations.

Try to segregate the growing fry(s) from the same lot gender-wise. Keep them in separate tanks, if possible. Alternatively, exchange them (either male/female) with mollies from a fellow hobbyist if possible, reducing the probability of inbreeding.

Hopefully, you will enjoy your journey as a molly breeder. We wish you a happy fish grandparenting time!

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About the Author

Shelby Crosby

Shelby is a passionate fishkeeper who has been writing about fish for over 5 years. She is a pro aquarist and holds a BSc Honors Degree in Wildlife and Fisheries. She creates her own beautiful aquarium layouts and loves to share her knowledge of tropical fish with other hobbyists. Career Highlights: Has worked with several…

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