Infusoria are any small, microscopic critters that appear in any rich culture medium. They are used to feed very tiny fry.
You can make one by boiling anything from crushed leaves, dead flowers, banana skins or potato peelings in water for a few minutes then leave to cool. A fistful per 40 litres is about right, according to Axelrod's Atlas. Leave the culture uncovered and let it ripen, aerating it and keeping it about 15 - 20C. If it is too bright you will get algae more than infusoria. In a few days the infusoria should be visible under a microscope or hand lens. To look at, the culture will look slightly cloudy.