How to breed swordtails

Swordtails are livebearers like guppies, but bigger, with bigger broods. A healthy female can give birth to 20 to 80 fry about once a month. Here's how to tell males from females, spot a pregnant swordtail, and raise the fry.

I've bred swordtails in my own tanks.

Males and females

• Males grow the "sword", a long extension on the bottom of the tail, and have a pointed anal fin (the gonopodium).
• Females have no sword, a fan-shaped anal fin and a rounder body.
• Late developers: some males don't grow a sword until they're several months old, so a young "female" can turn out to be a male.
• Ratio: one male to two or three females. Two males in a small tank often fight.

Is my swordtail pregnant?

She'll get a rounder, then boxy belly with a dark gravid spot near the back of it. Pregnancy lasts about 4 weeks. Close to birth she may hide or hang near the heater. Females store sperm, so she can have several broods without a male.

Saving the fry

Adult swordtails eat fry. The same three choices work as for guppies: dense plants like java moss and hornwort, a breeder box for a day or two around the birth, or a separate fry tank. Swordtail fry are bigger and stronger than guppy fry, so more survive in a planted tank. See how to breed guppies for the full method.

Feeding and growing fry

• Baby brine shrimp or finely crushed flakes, 3 to 4 times a day.
• Warm water, about 76–78°F, speeds growth.
• Room to grow. Adults reach 4 to 5 inches, so plan for a tank of 30 gallons or more. See swordtail care.

Swordtails and platies

Swordtails and platies are close relatives and can crossbreed. That's how many fancy colors were made. If you want to keep a pure line, keep them apart.

Related: Swordtail care · How to breed guppies · Breeding aquarium fish