Paramecium is a tiny (less than half a mm) little animal that is so small they aren't even easy to see with the naked eye, they usually appear like cloudy water.
They are fed to fish that would eat infusoria for the first few days (like pretty much any fry), you can culture them using some water on a windowsill with something rotting like bannana peel in it, they eat algae/bacteria that grow on the banana skin.
Although I've seen various cultures like Paramecium species for sale, I don't think there's any advantage from feeding just Paramecium as opposed to the multitude of different similarly sized creatures you would get from a standard infusoria culture.
And for fish big enough, baby brine shrimp and fairy shrimp certainly make them grow faster .