Infusoria is micro organisms in water. They can be cultivated as food for fish fry of small fish species. There are commercial processed foods that serves the same function as infusoria as food for tiny fish fry. They can be in liquid form as in dense liquids or paste, and in solid forms like micro pellets. However, feeding infusoria will be better than these commercial foods because fish always take live food more readily.
Infusoria are widely recommeded to be cultivated by using boiled lettuce. However, obtaining starter infusoria cultures to start the infusoria growing is difficult in some places. You can also take some green water and use it as a base culture to grow your infusoria from since green water always contain infusoria. However, you can also pick up nasties that preys/parasite on fish fry since theres no effective way to filter them out. Besides, if not successful, an infusoria culture gone bad can pollute your fry tank and kill them instead.
The best solution is to forget the infusoria culturing and put a seasoned unwashed sponge filter (in use for more than 2 months) into the tank. There is always lots of infusoria on the scum that the frys can eat plus its ready bio filteration can keep your water in control. It wouldn't be long before they will be able to start eating baby daphnia or baby brine shrimp.