Why do Swordtail Fish Eat Their Fry?

If you fail to care for your swordtail fry properly, they will get eaten by their parents or other fish. Like all the other livebearers, swordtails do not have maternal instincts and will, therefore, consume their young ones soon after birth. But why do swordtails eat their fry?


Swordtail Fish Fry – Wojciech J. Płuciennik (CC BY-SA 4.0)

Swordtails are peaceful fish species, but you will have to take several measures to ensure that your baby fish are safe until they get to the juvenile stage or big enough, that they will not fit in their parents mouth.

The movement of the small fry imitate live food. Swordtails and fish in general, are genetically coded to catch these small creature. Little they know, that these small moving creatures are their fry.

Another reason why the mother swordtails eat their young ones is hunger. If you provide them with the right food, they may not touch the babies.

Unfortunately, most of the cheap brand foods do not offer the nutrition your fish requires. Therefor your fish might be hungry, even if you feed them enough. By the way, fish are always hungry.

Below, I have highlighted some of the common practices that you can apply in order to safe swordtail fry from getting eaten by their parents or other fish.

Saving Swordtail Fish Fry

As I have stated above, swordtails lack mothering instincts. You will, therefore, need to safeguard the fry until they are very large to be eaten. One of the effective methods involves separating them from adults.

You might also add more floating plants in the tank to camouflage the young ones, but this method is less effective. Some people also prefer to use breeding traps to separate the baby fish from the adult ones. That may increase their stress levels leading to earlier births.

To prevent stressful conditions in your aquarium, you need to either remove the adult swordtails or the young ones – whichever is easier – from the aquarium and into another one. Also, remember to add more plants to the aquarium to allow camouflaging if you do not want to transfer them. Here are the measures you can take. Choose one that matches your condition.

Using a Breeding Box

To protect your swordtail fry, you can buy a breeding box from any pet store. The inexpensive item comes as a plastic container to hold your pregnant swordtail. However, you should not place the fish in it until it is ready to give birth because that might induce stress.

The breeding box has two compartments and it will float in the aquarium. Its top compartment will house the pregnant fish and the smaller compartment – between the bottom and top compartments – allow the fry to drop to the bottom compartment.

After that, the mother will not be able to reach the fry in the lower compartment. Even though it is very unlikely, the baby fish might swim to the top compartment through the space between the bottom and top compartments.

Therefore, after the swordtail gives birth, you will have to remove it from the top box and place it in the community tank. Moreover, you will have to remove the plastic piece that separates the bottom compartment from the top one from the breeding box so that the newborn can have adequate space for swimming.

Remember that the breeding box is not a good place to raise the fry for many days because the water is likely to become stagnant. You will have to replace the water with that from the community aquarium regularly if you want to keep the fry in the trap.

Fry are very small and you should raise them in a separate aquarium to prevent their swimming into the community aquarium tank where the adult fish can eat them. A breeding box can accommodate 2 pregnant females and provide a safer place for the fry after birth.

Use of Live Plants

If you would want to keep the fry in the community aquarium tank, you will have to add more floating plants in it to provide enough hiding place for the fry. Dense and bushy plants are a good choice.

They include the hornworts, guppy grass, java moss and the water lettuce roots, which improve the survival rate of baby swordtails. The plants will provide adequate cover for around two weeks after which is they will strong and big enough to escape from the adults.

Remove Adult Swordtails from the Tank

At times, it can be hard to catch the newborn fry. To avoid the hassle, you can remove all the adult swordtails from the tank and leave the fry to grow. That way, you will have protected them from any hungry adult. Moreover, this step will benefit the fry because it will reduce the waste in the water. You can use a mesh net to remove the mature fish from the community aquarium.

Certainly, if you keep swordtails together with other community fish, you will want to remove all of them, because they will also hunt down the small fry.

When is a Swordtail Ready to Give Birth?

The gestation period of swordtails is 28 days. However, it can be hard to know when they are pregnant. The gestation period also varies from swordtail to the other and that will highly depend on the level of stress or the water conditions.

After the swordtail gives birth severally, it will be easier to know the right time to remove her from the community aquarium tank.

Including after the belly has not started enlarging; you should assume that the swordtail is pregnant if she has been living with the males. Move any expectant mother to a small heavily planted tank when the swelling of the belly is noticeable. Alternatively, you can place her in a spawning box to keep the fry alive after its birth. Frys have to hide until they are mature.

Scrutinize the pregnant swordtail body on a daily basis as the time to deliver nears. The bottom of the swollen belly will appear slightly square and will look very light – more as she might burst. Dark large spots appear on both sides of the swordtail near the anus.

The gravid spot will help you see its uterus pressing against its abdominal walls. The time is right after she stops eating, stops swimming throughout the tank lie down at the bottom or stays at the back of the aquarium tank.

Related Questions

When does swordtail fry start eating?

The fry will start eating immediately after birth. Baby fish accept most of the available fry food but they are not large enough to consume the regular flake food.

If you do not have fry food, you can grind flakes into powdered form and use the powder as the main feed. Feed them with baby brine shrimp, live foods and micro worms to ensure that they are healthy. A fry might become obese if they continue eating throughout the day. Furthermore, they hate older food.

Provide them with food that they can consume within 3 minutes. And if you prefer frozen food, you will have to defrost them before you drop them to the aquarium. That way, you will eliminate the chances of the frys ingesting hard food.

At what stage can you put swordtail fry together with their parents?

To introduce the swordtails in the community aquarium tank, you will have to sex and separate them first. Do that as early as possible. To determine the sex, check the anal fins also known as gonopodium in males situated on their underside.

Males feature long pointed anal fins and those of females are more rounded and fan-shaped. Separation of the males from females will make them more stressed and as a result, they will fight for dominance. After a fry attains the age of 6 months, it is ready for breeding and you can put it in the community aquarium tank. Keep over 3 females for every one male.

Swordtails do not change sex. The baby swordtails will take a longer time to show their sex. They all look like females but the males will start developing the gonopodium and the long tails later in life. That takes place in two different stages.

At times, the fry will show male qualities at a younger stage but will end up with a small thin body shape compared to the other swordtails. People mistake the late developing male swordtails for females, but you should not do that. They are usually heavy bodies than the early male swordtails and become the dominant males after developing their male qualities.

At what age should you remove swordtail fry from the breeding box?

For the fry to grow faster, you will have to provide them with enough space by introducing them into a larger tank, adding cleaner water of moderate hardness into the tank and providing them with more live food. That will facilitate faster and healthier growth while ensuring bright coloring.

During the growth stage, you should never keep the fry in traps because they do not offer enough space to facilitate a comfortable increase in size. In fact, they can stunt their size. Keep the fry in a separate tank until it grows up. You will have to wait for 2 months for them not to fit in the mature swordtail’s mouth. At that stage, you should introduce them to the community tank.


Due to their peaceful nature, swordtails are ideal for a community aquarium. The first few months of the fry will play an important role. Good water conditions in addition to quality food will facilitate the development of healthier beautiful swordtails.

Breeding and raising of swordtails is fun and doing it for a profit is another story. For proper growth, keep the lights on for around 12-16 hours in a day. Bright or strong light is not important but it will help avoid deformities in the spine. Fish also want to rest, which means that you have to switch off the light from time to time. Give them 6-8 hours of darkness.

Swordtail Fish   Updated: October 7, 2019
avatar Hey, I'm Fabian, chief editor at Aquarium Nexus. I really enjoy the aquarium hobby and love sharing my experience with others. If you have any questions feel free to contact me or leave a comment below.

Leave a Comment

Your email address will not be published. Required fields are marked *