How to Tell If a Swordtail Is Mating?

Swordtails, Xiphophorus helleri, are peaceful fish that do well in a community aquarium and typically get along well with most other fish in a community. They come in a variety of colors including orange, green, red and yellow. These fish are easy to raise as long as you provide protection for the young. The first step in raising swordtails is to make sure your fish are mating.

Determining the Sexes

You need to have at least one male and one female for mating to occur, so checking the sexes of your fish is a good place to start. Males are easy to distinguish from the females, since they’re the ones with the distinctive “sword” protruding from the lower part of their tails. They also possess a gonopodium, a long, thin modified anal fin used for mating. Females lack the long sword tail, and the anal fin on females is stubby and triangular.

Watch Their Behavior

Swordtails mate very quickly, so if you aren’t watching them you could easily miss it. Typically the male will approach the female, often from the side, and may flare his fins in an attempt to impress her. He then swims up close to her and quickly touches her on the underside, near her anal fin, with his gonopodium. It only takes a moment to transfer the sperm so he won’t linger, but he’ll often return to her again and again as he repeatedly attempts to mate.

How Many Swordtails to Keep

Male swordtails may fight with one another in their attempts to secure territorial and breeding rights within the tank, so it’s generally best to have only one adult male in your aquarium. A male won’t fight with a female, but since he can be persistent when attempting to mate, he may overtire his companion if he’s housed with a single female. For this reason it’s best to have a single male and at least two or three females. Typically he’ll manage to mate with all the females in his harem, and none of them will be overwhelmed by his attentions.

Definitive Proof

Ultimately, you’ll be able to tell if your swordtail has been mating when babies, called fry, appear. The female swells as fry grow inside of her, and the dark-colored gravid spot by her anal fin becomes obvious as the young develop. About four weeks after mating she’ll give birth to as many as 80 live fry. Since female swordtails can store sperm, she can continue to produce fry for months after a single mating. Keep your fish in an aquarium with lots of plants or separate females before they give birth to prevent the adults from eating the young.