Baby fish eat brine shrimp. It's as simple as that.
Fortunately, baby shrimp are very easy to hatch.
I started out with this brine shrimp hatchery. You pour a packet into the black part and fill it up with water. Then you fill the little clear "glass" (really plastic) with water and invert it on top of the black piece. In less than 24 hours, the shrimp begin to hatch. As they do, they swim into the clear part to be closer to the light. At any point, you can remove the clear part and dump the brine shrimp and the water into the baby's tank. They eagerly swim after them.
The brine shrimp are about the size of the head of a pin.
Because I have so many babies right now, I bought a two-gallon tank and filled it with water. Now I add a spoonful of shrimp eggs and a larger spoonful of aquarium salt. Within 24 hours, I have literally thousands of baby brine shrimp. I dip a special cup (an old betta cup) into the tank and gather up several hundred and dump that into the tank with the baby angelfish.
The angelfish need to be fed about four times a day because their little tummies can't hold much at a time. If they are fed less often or not enough food, the weaker ones will starve.
I use San Francisco Bay Brand, which is known as a quality product.
Very soon I will begin feeding the two-week old angelfish some tiny flecks of dry fish food so by the time they are sold to the pet store, they will be well prepared for someone's aquarium.
