Summarize:

Platy fish are relatively easy to breed, and they don't require a lot of encouragement. As long as you introduced both male and female platy fish to the tank, they should start breeding in a short amount of time. If your platy fish don't seem to be breeding, make sure you have both male and female fish in the tank. Also, make sure your fish are healthy and that you're feeding and caring for them properly since unhealthy platy fish might not breed. Female platy fish will develop a dark spot on their abdomen when they're pregnant. Try to get in the habit of checking your female platy fish daily for these dark spots. Then, if you discover that one of them is pregnant, write down the date so you're able to keep track of how long it's been pregnant for. This is the average gestation period for platy fish. If you wrote down when you first noticed your platy fish was pregnant, the fry should be born about 24-30 days from that date.  If you're not keeping track of when your female platy fish get pregnant, it can be hard to estimate when they're going to give birth. However, it will always be within 30 days of when you notice they're pregnant. Don't worry about doing anything special for your pregnant platy fish during their pregnancy or when they give birth. As long as you're feeding and caring for them properly, the pregnancy should go smoothly. once they're born if you're worried about them being eaten. If you've provided enough hiding places in the aquarium, like shelters and floating plants, most of the fry should be able to hide from the adults as they grow. However, if you don't want any of the fry to be eaten, you may want to transfer them to a new tank or set up a breeding trap in the tank to keep them separated.  You can move the mother to a separate tank before it gives birth, or you can separate the fry after. To move the fry, use a fishing net to scoop them out of the tank and transfer them to a new one. Depending on where the fry are hiding, you may need to scoop up a shelter or floating plant in order to catch them. Baby fish food usually consists of pulverized flakes that are easy for the small fry to eat. You should also supplement their diet with baby brine shrimp, similar to how you supplement your adult platy fishes' diet. Feed the fry in small amounts several times a day like you do with your adult platy fish. Since platy fish can quickly overpopulate an aquarium, it's important that you have a plan in place for what to do with the fry as they grow and mature. Platy fish fry reach sexual maturity at 4 months of age, so you'll either want to give them to someone else or move them to their own tank before they reach 4 months. Otherwise, your tank will quickly become overcrowded, and your fish will suffer and get sick. Consider selling your platy fish fry and turning it into a small side business. Or, you can give them to friends who are also aquarium enthusiasts.
Wait for your platy fish to start breeding after introducing them to the aquarium. Check female fish for a dark spot to determine if they're pregnant. Wait 24-30 days for pregnant platy fish to give birth. Separate the fry Feed the fry commercial baby fish food and baby brine shrimp. Sell or giveaway your platy fish as you breed them, or move them to a new tank.