Once they are free-swimming and their yolk sac is gone, you'll start feeding. But more like green water, egg yolk clouds and liquifry at first. I think it will be a few days before they are big enough for BBS.
There are two stages within the first week and a half:
WIGGLERS and FREE SWIMMING
Right after hatching, they are labeled wigglers because, well, they wiggl and do not swim.....And once the yolk sac is absorbed, the fish will be within a day or two of becoming free swimming.......When they start to levatate(aka-try to swim) this is the time to start the spawn on bbs.........
My convict frys started eating after about 3 days from hatching. They picked up algae or whatever is in the tank. I noticed that the mother crunched up the flakes and pellets for the babies.
The store also sell the fry food in a tube or you can put boiled lettuce in a jar out in the sun and feed them the infusoria.