Welcome to the hobby and congrats on being a successful fish breeder already!
Yep, livebearer females are often already gravid when you get them, you're not alone! I really like the look of the tank so far, the wood, dragonstone, substrate and plants look great!
Just some quick questions since these are your first fish - do you know about the nitrogen cycle and have you cycled your tank? Do you have a water testing kit?
As to your question about fry, it's very likely that at least a couple will survive. But there are things you can do to increase the odds! More plants is the key to having lots of babies survive - enough that you'll soon have more fish than you know what to do with, and you regret ever getting livebearers...
(Kidding! Kinda)
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Out of all the platy varieties, the blues are my favourite! Such stunning colours, and a variety of blue speckling. These ones are the lighter blue ones from my original (huge) female in the first pic, but after some generations I have a gorgeous male that has really dark speckling all over in a really pretty pattern. Need to get some photos of him, but he's also super shy and hides away in the plants.
You can definitely raise the fry in the tank with adults, without worrying about breeder nets or boxes. Personally, I think those do more harm than good. Adults will pick off weak, slow, ill fry more easily, so the healthier fry that can dash to a hiding place tend to survive. Well fed adults are also less inclined to put in the effort to hunt down down fry; but overfeeding should also be avoided of course. It's all about balance.
The bottom of your tank does provide some hiding spots, but the middle and top sections of the tank are wide open. That's not so good. Consider that livebearer fry in the wild are born in rivers, and fry instinctively head for the shallower edges of the river when they're born, because the middle, deeper part of the rivers is where the predators and adults are. They head for the surface and the rivers edge to hide among the vegetation/roots/rocks etc, only venturing away from this "nursery" as they get bigger. So newly born livebearer fry in our tanks also tend to head to the surface (and some down to hide at the bottom, but that's less common in my experience). Having floating plants up there gives them safety and security, and a really dense bunch of something like elodea or guppy grass is ideal. Any fast growing stem plants that can grow while floating at the surface will work. The more dense the bunches of plant, the easier it is for fry to escape an adult fish, which can't navigate through the plant as easily as the fry.
To show you what I mean;
I used to raise platy and guppy fry in this tank, and I always recommend planting that covers the entire height of the tank in at least one area, if you want most of the fry to make it. That thick, bushy plant on the left is limnophilia sessiliflora. Easy and fast growing, it'll reach the surface and keep going. Can trim the tops off and replant them, making a really dense thicket of plant that provides hiding spots for the entire height of the tank so that no matter where a fry is, it can quickly escape a hungry adult! A thicket of dense plant like this can be created with most any fast growing stem plant, and the fry born in here could always be found hiding in there, or among the roots of the frogbit at the surface.
By the time fry are about 3 weeks old, they'll be big enough that the adults won't eat them.
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As for feeding, they'll be just fine on the same food as the adults. If you're using a flake food, crush it so it's more fine and easier for tiny mouths to manage. They'll be fine if you feed them once or twice a day, but fry tend to do better when fed little and often. Small amounts of food given 3-4 times a day worked for me, when possible, but it's not essential to feed that often, and it's all too easy to overfeed.
Increase water changes when you have growing fry, since more fish and more food means more waste in the tank.
If you feed small foods that tend to hang in the water column for a while, you can feed the adults at the opposite end of the tank from the 'nursery' plants where the plants are hiding, then target feed some finely crushed small flake or other food in among the plant for the fry. Then they don't even need to come out of hiding to eat!
Bug Bites is a great, healthy food, and the smallest size (light blue tub, microgranules) are relished by adults and fry, and tends to sink slowly, giving them a chance to eat at the surface and in the middle of the water column. Foods like daphnia, cyclops and baby brine shrimp form a cloud in the water column, giving every fish a chance to eat without fry having to venture too close to the adults.
Please do keep us updated, and feel free to ask for any clarification, or any other questions you have.
