It depends on your setup.
If your tank is established and heavily planted, it will be full of infusoria (tiny critters that feed on plant matter), which your fry, if left at large in the tank, could feed on happily until old enough to compete for flakes (cultivating infusoria, youngsters, was what we all used to do in the days before the coming of liquifry).
If your tank is heavily planted and not full of dedicated fry eaters (such as angel fish), it might well stand a good chance of survival left as it is. I have a number fry who were born when I was on holiday and seem to have fended for themselves quite happily. You can always add fryflakes or crushed flakes to the main tank; even if the adult scoff most of it it won't do them any harm.
If your tank is not planted and/or full of rpedators, then the breeding trap is the best option and then of course you will need to feed it. Be careful not to overfeed, as it is only one and you don't want to foul its living quarters. Sticking a plant in is a good idea, for shelter, infusoria and secutiry.
Hang on- why am I writing all this? nmonks has just stuck a brilliant thread in the livebearers forum which says all you need to know much better than I could. Check it out!
