Infusoria - google it and you will find various recipes. Basically they are tiny organisms which live in water on decaying vegetable matter and are small enough for the tadpoles to eat.
My son is feeding his African Dwarf Frog tadpoles Liquifry No.2 made by Interpet. One drop per 4 litres of water 2 - 4 times a day. The first few days I think they feed on their left over yoke. But his tadpoles fed on this stuff right away! Hope this helps.