A breeding net is probably the best answer as Lenny said as it means they're in a cycled tank. If you want to put them in the small tank then a very basic
sponge filter like this is fine for fry. You'll need an airpump to run it. If you move them into the spare tank bear in mind that as the sponge won't be cycled you'll need to do daily partial water changes.
Best way I've found for catching very tiny fry is to suck them up with a
turkey baster.