Try not to panic. I know it feels really emotional when it's your first time getting fry, but your female platy will have enough sperm stored to keep popping out fry monthly for the next year or so. Believe me, soon enough, you'll have more platies than you know what to do with, even in a community tank! The fry still stands a chance, even if you don't catch it. There are zebra danios, a gourami, and several huge livebearers like mollies in my community tank, and I have so many platy and molly fry survive. My other tank has adult mollies and almost adult platies, and a guppy fry I couldn't catch has grown up in there and is now big enough to avoid being eaten, he's doing just as well as his 37 siblings that are in the grow out tank, and plenty of other fry have been raised in that tank with the adults
If you really want to, you can catch the fry. Set up some buckets with tank water in them, feed the fish, scoop the adults out and put them in one of the buckets. Then use two nets - one to chase, one to catch, and catch that fry! Don't worry if you make a mess of the tank, you can tidy and water change afterwards. Just be patient, try to get the net underneath the fry and scoop it from below. Generally what works for me. I left the last fry I couldn't catch recently only because I'd already uprooted too many plants catching the others, and the tank is densely planted so I knew he had a good chance anyway.
If you catch him, he can go in the breeder box with the older fry, since it's only a couple of weeks old, it's still too small to eat the newborn. If you made a mess of the tank, tidy it up and do a water change, cleaning the substrate, before adding the adults back. Making sure replacement water is temperature matched to the tank water and that you use water conditioner.
If you want to try leaving him in the tank, the more dense planting you can add, the better, whether real or fake. Floating plants with long roots like frogbit and water lettuce are ideal, as are stem plants that you can leave floating like guppy grass, elodea, water wisteria, or hornwort, among others. Livebearer fry instinctively want to go to the surface to hide, since in rivers they would go to the shallow edges of the river to avoid the adults and larger predators, so plants at the surface are great.
I like my limnophilia sessiliflora for fry too... it grows fast, and provides really dense hiding spots that adults can't navigate as quickly as the fry, and it reaches to the surface quickly too, so no matter where the fry is in the tank, it can dive for cover. (the thick plant on the left at the back, it's where my single fry spent the first few weeks)
Also don't keep the fry in the breeder box for too long. Once they're 2-3 weeks old, they're large enough to avoid being eaten, and start coming out into the open more. And adult might try to chase them now and then, but they seem to recognise that it's a fish, not food, and too big for them to eat and give up the chase. Keeping them in the box too long can stunt their growth. If you can have another tank as a grow out then great, move them straight there rather than in a breeder box, but otherwise, once 2-3 weeks old, as long as your other fish have small enough mouths not to be able to eat them, put them back in the main tank.
Key to keeping fry alive in a community tank:
Lots of cover, dense planting to hide in.
Crush food really fine so it's small enough for their mouths. Crushed flake food is fine, as is the microgranules Bug Bites (very popular with all my fish, adults and fry alike) and frozen food is great, since it floats about it clouds all over the tank if you melt the cube into the tank and spread it around, cyclops, daphnia, moina, rotifers - all small enough for fry, enjoyed by adults too, and means the fry don't have to come out of hiding to eat, and stand more chance of not being eaten.
Keep the adults well fed, without overfeeding too much and turning the water toxic. Well fed adults are lazy, and less likely to put in the effort to chase down fry. Feed little and often, 2-3 times a day is fine if you can do it, but twice a day is fine too.
Up the water changes. More food in means more ammonia produced, so monitor your water with testing, make sure ammonia is zero, nitrites are zero, nitrates below 20ppm. If any ammonia or nitrites, 75% water change. Water change 2-3 times a week, depending on the size of the tank and the bioload, more is better than less, especially with growing young fry, which grow better with more frequent water changes.
Sorry for the essay, happy to answer any more questions!
