You'd probably have to feed them some baby brine shrimp or maybe some liquid fry food.
I think if there is enough cover, that you could leave them in the tank. I'm sure there are micro organisms in there that they could eat on. If you get some baby brine shrimp you could toss those in as well.
I don't have experience with egg layers fry, so I'm sorry I can't give you a ton of info.
I did find a Celestial Pearl Danio fry once and moved it to another tank and wished I had left it alone.
I think it would have lived longer in the tank.