Platys are livebearing tropical fish, which means they give birth to fully developed young, rather than eggs that will hatch later. Because female platys are capable of storing male sperm for weeks after mating, knowing how to tell the difference between the sexes can help ensure that your tank does not become a full-time fish nursery. Luckily, sexing a platy is not difficult.
Find the Fins Edit
Look closely at your platy's underside, and note the two fins protruding from its belly. The fin in the rear, or anal fin, is called the gonopodium. In female platys this fin is shaped like a fan. If your platy is male, however, the gonopodium will be pointy, flat and long.
Sex Your Tank Edit
Keep all female platys in a separate tank from males unless you are planning to breed your fish. Male platys may exhibit aggression toward other males, so if you intend to add to your school periodically and have no intention of breeding, fill your tank with females.
Mixing Sexes Edit
Populate your tank with no more than one male platy to every two females if you are planning to breed your fish. This will reduce the likelihood of aggression between the males and stress in the females.
Aesthetics of Sex Edit
Add male platys to your female tank if you prefer brighter, vibrant colors for viewing, but keep in mind that doing so will result in offspring. Consider instead introducing other breeds of male tropical fish, such as mollies or guppies, if you prefer a colorful tank but are not interested in breeding your school. Avoid aggressive species like bettas who might start fights.