Green water and/or infusoria were once used extensively as food for extremely small fry, especially for species that need to grow a bit before they can handle newly hatched brine shrimp. Another trick sometimes used was to hardboil an egg, place the yolk into a piece of fine cloth, and squeeze it so that minute fragments are released into the water. Mature sponge filters are good for fry as well, not only because they are the safest filter to use with tiny fish but also because the multitude of micro-organisms that grow on them provide a food source as well.
All these ideas are usually only needed for a couple of days until the fry grow large enough for baby brine shrimp. And all of them are liable to screw up your water quality quickly, so extensive water changes, often daily, are used to prevent this.
