The ice cube floats on the surface and the bbs comes down in a stream. Larger fry will learn to attack the ice cube as soon as you drop it. Most importantly, fresh bbs is always on hand when you get that surprising spawn. Except for discus, I usually move spawns to a small tank to hatch and raise. After that stage, however, the rate of attribution increases rapidly.
Young fry are easier to feed in small tanks.
However, it's much harder to maintain constant temperature and good water quality. Also, when the hatch is big, it's hard to make water changes without shocking the fry. With heavy feedings, however, large water changes are absolutely necessary. High ammonia and nitrate levels lead to high mortality levels. The obvious approach I took was to put the small tank in a larger tank with its own heater so that it's easier to maintain constant temperature.