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Like other livebearing aquarium fish, swordtails (Xiphophorus hellerii) give birth to live young, or fry, rather than laying eggs. Although the swordtail’s gestation period is 28 days, it may be difficult to determine when she became pregnant. Gestation can vary by a few days if the female is stressed or water conditions aren’t quite right. Once your swordtail has birthed several times, you’ll be able to tell when to remove her from the general population.
Even before her belly begins to enlarge, assume your swordtail is or soon will be pregnant if you keep her with males. Move the expectant mother to a separate, smaller, heavily planted nursery aquarium when her belly swells noticeably. Alternately, put her into a false-bottomed spawning box in your current setup. The fry are considered food by all the other fish in the tank, including their mama. They’ll need to hide until they grow too big to be eaten.
Scrutinize the pregnant female’s body daily. As her time nears, the bottom of her swollen belly will begin to appear slightly square and she’ll look light she might burst. A large dark spot will develop on each side of her body near the anus. That “gravid spot” is where you can see the uterus pressing against her abdominal wall.
Her time is at hand when the female loses interest in food, stops swimming around, hangs out near the back of the tank and lies down on the gravel.