As they've grown, however, and especially once I removed the plants they were hiding behind, they have become more confident with their surroundings. They've begun to swim from the bottom of the tank to the top. And they've fanned out, no longer feeling the need to remain in a school.
In the wild or in a community tank, plants or decorations with very small openings are necessary to keep the baby angelfish from being eaten by predators - even other (adult) angelfish. Remaining in a school can be necessary for survival as well.
But in an infant tank like I have set up, there are no predators. I don't even have corydoras or other bottom-feeding or algae-eating fish, because they can find newborn fry very tasty. So there are no predators and the angelfish are learning they are perfectly safe.
I've been asked about the sponge on the intake filter. I bought the sponge at my local pet shop and cut it so it slips onto the intake. This prevents very small and weak angelfish from being sucked into the filter. It also provides a place where brine shrimp can become lodged, which the babies will pick off without having to worry about getting sucked in. The water can still circulate through the filter, providing both filtering and water movement that is essential to keeping the water oxygenated.