fry take 3 days to consume their yolk sac, then when they swim vertically they start to eat. so if I was going to add infusoria it would probably be 3 days after they are wriggling, because any sooner and they may die before the fry can eat them (who knows?) and any later and the fry will hopefully have had other food and will be big enough that infusoria may not be a great deal of use to the,. But I have always relied on MW, VE and the internal infusoria population for first foods, rather than culturing it. Maybe someone else has actual experience re this?