Many young fish are so small when they first hatch that they can not eat traditional 'first foods' such as newly hatched artemia, microworms and vinegar eels. Amongst the nothos, this is particularly the case for N. janpapi, N. luekei and N. geminus - the Aphyobranchius subgenus. Other Nothobranchius, particularly from young parents, can also have similar problems (N. rachovii and N. patrizii spring to mind).
Infusoria is the collective name for microscopic organisms that dwell in bodies of water, feeding on detrietus and smaller single celled organisms, and include rotifers, and paramoecium. These micro-organisms live amongst plants, gravel, and in the filter, in fact anywhere they can. Squeezing out a sponge filter will yield a large number of these infusorians. For most (non-surface dwelling) fry it is ok to simply let them grow for a few days in a well matured tank with a sponge filter, or with a large clump of javamoss. A few days on and the fry will be eating newly hatched artemia nauplii - so why bother to raise paramoecium?
The smaller nothos (Aphyobranchius species) are surface dwellers, the newly hatched fry spending their first days living at the very surface of their tank - often in the meniscus around the edge. Because of this tendancy to remain near the water surface, the infusoria associated with plant matter is unavailable to the young fry, and can lead to starvation. paramoecium, on the other hand will stay near the waters surface if the tank is illuminated from above - exactly where the young fish need them. Another method is to have a slow air bubble (1 bubble/second) going to the surface (and bringing the paramoecium up without causing excessive turbulence.
Paramoecium are very easy to keep; as long as their water is clean, aerated and free of chlorine, they will thrive. I use aged tapwater. They can be fed easily with the smallest quantities of evaporated milk - just enough to give their water a slightly cloudy appearance. Collection is also relatively easy; if the culture is poured into a tall thin bottle, such as a clear wine bottle, the paramoecium can be seen congregating around the water surface, where they can be easily removed. This is not really necessary though as the culture can be simply poured (in part) in with your fry. Some people even add a tiny drop of milk in with the fry, just to get the paramoecium really thriving with the fry.
The greatest problem is with contamination - if any tank water gets into the cultures they will soon be outcompeted by other infusorians. Failure to feed them is not a major problem, as they will return even if unfed for many weeks.