They should be able to pick at the leftovers int he tank. Either that, or a small grain pellet or a flake food should work.. Some go so far as bably brine shrimp for their fry.
We have some frozen stuff, dont remember what it is called as my wife bought it. SUPER fine. So, who knows if they ate it. We tried the ground up flake because it would be easier to see.
The babies came out without the egg sack already. They are much larger than we expected them to be.