Platy fish are relatively easy to breed, such that they – and guppies and mollies – are frequently joked about, that any female who has met a male will surely have babies.
Male and female platy fish are easy to distinguish, from as early as two months old. While a female will maintain stereotypical fan-shaped fins, the male anal fin develops into a thin tube for delivering sperm directly to females. The tube is not entered into the female, acting more like the tube of a dart gun than a syringe. It does work quite well, as long as the anal fin has developed properly.
A female that has received sperm can produce fry every month for a few months. While it may take a few moments to straighten out and flex their muscles, fry are free swimming from birth. This is why platies are considered “live bearers”.
Ensure feeding is consistent and water parameters kept in safe ranges. Stressed fish have a harder time staying fit while breeding and females may die from giving birth if they aren’t kept healthy. Fry are more likely to end up as fish food, too, if the adults are regularly hungry.
As long as a male platy gets a chance to greet a female, it only takes the greeting for fertilization to happen – presuming both are fertile. You may be inclined to leave the fish together a few hours just to be sure, but if a day isn’t a enough, longer certainly wouldn’t help any more.
It’s takes approximately one month for fertilized eggs to develop to the point of being ready for birth. The timeline and fry count may vary though, depending on fish health and environmental conditions. Fish kept in an ideal situation may be more prolific, while others take longer to produce fewer fry. Some fish just don’t breed well in ideal conditions too.
Fry need food as much as the adults and likewise should receive multiple small servings of food each day. Many adult dried foods can be crushed into fry-sized portions, but many people suppliment with live food. Brine shrimp, daphnia, and vineger eels are common live foods.