How Long are Swordtail Fish Pregnant For?


Swordtail Fish Gestation Period

If you keep male and female swordtails together in the same aquarium, most likely the females will get pregnant. This is a natural process that also happens in the wild.

If you are into breeding swordtails, you might want to save as many fry as you can. In this case you have a few options.

I usually keep the females together with the males for one week and after that I will separate them and place them in a heavily planted nursery tank. I understand that not all hobby fish keepers have this option.

In this case you can separate the female just before giving birth. But how do you know when is the time of the delivery for a pregnant sowrdtail fish?

The gestation period of swordtails is about 28 days, but the delivery can delay due to bad water conditions, stress or other factors.

So I would recommend separating the female from the others just before the end of the gestation period. This way you can ensure that most of the swordtail fry will be safe.

Giving birth already puts a lot of stress on the female swordtails. So try to place the female in the same water conditions as she was before in order to avoid further stress. The water temperature should be between 70-82 °F (21-28 °C). The water pH should be between 7.0 and 8.3 while the water hardness should be between 12 and 18 DH.

Because swordtails are really social fish, you should not separate the pregnant female for too long from the rest. After she gives birth, but her back in the main tank and let the fry gain power and start swimming.

I’ve wrote a complete guide on sowrdtail fish fry care, which you should read, if you are into breeding and raising swordtails.

Determine if Swordtail Fish is Pregnant

If a swordtail is pregnant, you will start seeing the belly becoming big, but to be sure that they are ready to give birth, you should check for other signs. A female that is almost giving birth will separate itself from the others and will get more aggressive towards the males. It will also hide in a corner and limit its movement within that specific area.

Swordtails get pregnant more often. That is the reason you will hear most people say that the only thing you have to do is to add some water in the aquarium tank. That is an exaggeration but keeping females and males in one tank means that you will have to care for new swordtail fry many times in a year. A swordtail fish will get pregnant immediately the give birth and they use the tendency.

You might feel like your swordtail is always pregnant and as a result, you forget how non-pregnant swordtails look like. Each new fry will mature within a short time to contribute to the population of your aquarium too – that will take around three months. So, if you are not careful, a few swordtails can overpopulate your aquarium tank.

Prevent Swordtail Fish From Getting Pregnant

One of the things I like seeing is new fry every few days. It makes me very happy. But due to various limitations, I have to take several control measures. It is hard to allow a swordtail to breed very fast, particularly because my tank is small and my budget is limited. So, to prevent unplanned pregnancies for my swordtail, I prefer keeping the females in separate aquarium tanks.

If the need to breed them comes, it is usually easier to introduce males to the tank because they are less. Swordtails are peaceful and segregation does not cause any aggression or stress. From my experience, the males appear to be more aggressive when a female swordtail is present.

That is because they harass females as they try to mate with them more frequently. If the idea of buying two separate tanks does not sound economical to you, you will have to buy a male swordtail and sell it after it has impregnated the females.

Common Causes of Delayed Birth

The gestation period of a swordtail is 28 days, but most of the times, the delivery might extend for around two or three days. If this happens, particularly if it is longer than one month, you should always know that something is wrong and you have to make changes in the aquarium tank.

Mostly, the problem is usually a result of unfavorable water conditions or a smaller size of the tank than the fish requires. Veterinarians and store dealers recommend a 20-gallon aquarium tank for swordtail fish along with heavy planting. They also advised me to ensure that the water temperature is around 64-83 degrees F and a PH between 7.0 and 8.3 and water hardness between 12 and 18DH.

Stress can also delay the delivery of pregnant swordtails. One of the key causes of stress is transferring the swordtail from one tank to the other more often or secluding her from the others.

If changing of the water conditions and reducing the stress does not offer any results, then the swordtail is not pregnant at all. I have heard many people claim that the gravid spots that develop on pregnant swordtails are fry’s eyeball. That is a misconception. It might be the uterus hue, which exists in every swordtail.

Related Questions

Should you move the pregnant female swordtail into a new aquarium?

A pregnant female swordtail fish might not have a large belly, and it is, therefore, good to assume that your fish is pregnant if you have been keeping it in one tank with the males. Remember to move every pregnant swordtail to another aquarium tank and add many plants when the belly swells noticeably.

Alternatively, you can place her in a spawning box in the community aquarium tank. All the fish in the community aquarium, including the mother, see the fry as food for themselves. Therefore, you have to provide them with a good hiding place until they have grown.

Scrutinize the pregnant females on a daily basis. As the time to give birth near, you will notice that the bottom of the swollen belly will start appearing square and the belly might look like it will burst. A large dark spot will appear on each side of the swordtail’s body close to the anus.

The gravid spot shows you the uterus, which presses against the swordtail’s abdominal wall. You should transfer the female to another tank or to a spawning tank after she loses interest in food, starts hanging near the bank of the aquarium tank, stops hanging or lies down at the bottom of the tank.

How many fry can you expect from a swordtail?

Like other livebearers, a swordtail can deliver 5-80 babies at a time but the average number is 30-35. They get pregnant immediately after giving birth and they will give birth after one month.

If maintained and fed properly, a swordtail can produce babies each month if there is a male in the tank. The swordtail fry measures around 5-7 mm (1/4 inch) in size and their size is close to that of guppy fry. They can also grow to around 38 mm (1.5 inches) within three months.

At that time, they will be mature enough for breeding. Even though they are very small, you will be able to spot them easily after birth. If you have eye problems, you will need a magnifying glass to see them.

How many times can a swordtail fish give birth?

A swordtail will start breeding after it is three months old, which means that it is good you separate them according to their sex before they attain that age. If the water conditions are right – the water hardness, PH, ammonia, nitrites and nitrates – your swordtails will breed regularly.

If the water condition is within the right levels, the stress level will be low and the spawning will be high. A swordtail can produce 4-6 times after the first birth.

According to experts, the second birth session may take up to six months and it can take place, including after you have separated the females from males.

Swordtails are able to keep sperms inside them for around eight months and use them for fertilization. And because the fish can eat their young ones, you should feed them properly when they are closer to giving birth. Remember to feed them at least two times a day.

Also, you should transfer the fry from the main tank after birth to save them from the hungry fish. The size of swordtail fry is similar to that of a point of your pen and they are clear as bubbles. When buying your fish, it is always good to check how the fry looks like so that you can know when the fish has given birth.

The other method of protecting the swordtail fry involves the introduction of more live plants into the aquarium tank to provide them with good hiding places. Stones and driftwoods will also offer a good hiding place.


Segregation of the fish according to their sex might sound like a perfect idea, but swordtails will always surprise you. Some people claim that their female swordtails change into males after they keep them alone in a separate tank. However, experts and researchers do not agree with that.

They claim that those are the late maturing males, who develop their male genitals – known as the swordtails – very late in life. But the tales of swordtails breeding in the absence of males are true. The females can store sperms inside them for up to eight months and they can get pregnant multiple times after single insemination.

The other interesting fact about swordtail fish is that they can cross-breed with  platies because they are from the same family. I’ve seen really interesting projects, where people also tried cross-breeding swordtails with guppies and mollies, but in realty I’ve never seen such a hybrid.

Questions and Answers

My female swordtail is getting quite big. She doesn’t have a dark spot but looks like something white is coming out of her… is that her getting dilated?! Or something else.

    If the poop of your swordtail fish is white, it can be a sign of disease. Female swordtails will also produce white poop before giving birth, but usually it is a sign of stress or disease. If the fish looks bloated and the scales are forming a spine cone shape, it might be a sign of dropsy. A picture would help a lot to identify the problem.

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