I don't have any spare filter / heater and I looked for storage box long ago and couldn't find any that didn't have any holes. I don't know if I can convince my mom to let me get the 23L back from the attic.You could get one of those large storage boxes for them, with a sponge filter & heater they'd be ok until big enough to go in the main tank.
Also if your main tank is well planted they're very good at hiding
The ones I've saved weren't good at hiding at all, they sat on the sand and did nothing. Would have been lunch long ago.
The only ones I see surviving are in the plant and are small, probably going to end up as males.
Basically these fry are either doomed to get killed in the main tank or if daily water changes won't be enough I'll lose them, but nothing much I can do, at least they get a higher chance to survive. One of them though might be best to feed him to the other fish, his tail is missing or bent, probably got bit. -.-
And my tank isn't well planted, I just have 1 plant in a basket that houses 2 fry so far and is frequented by all the other fish though and a tiny branch of it. It's a water sprite.