Newly hatched fry from many fish species require a diet of infusoria before moving on to brine shrimp nauplii - but what is infusoria and where can I get some?
Infusoria is the collective name given to the plethora of microscopic organisms that dwell within a body of water; they hang out amongst plants and the detitritus at the bottom of the tank; amongst gravel, and particularly decaying leaf matter. An estabalished aquarium will be rich in them already. The film below shows the a large rotifer, several smaller paramecium and other protozoans. This was actually collected from drop of water in an old bucket left outside for a year or two, with a good layer of rotting leaves and other garden waste at the bottom.
The still black areas are the fragments of debris amongst which the organisms live.
One way to feed infusoria to small fry, is to release them into an empty, but well-established, tank; another is to add plants - maybe epiphytes like java moss and java fern - from another tank to the rearing tank. More controllably, pure paramoecium cultures are available. These can be fed to fry in an otherwise empty rearing tank, with the very lightest aeration - a single bubble, every second or two, just to keep them circulated.
Culturing paramoecium is actually really easy. They may be kept in lidded glass jam jars at room temperature. Occasionally add a tiny drop of evapourated milk - and over the next days a cloud of minute paramoecium will replace the cloudiness caused by the milk. These can be added directly to a fry tank.