From The Aquarium Wiki
What are they
Infusoria is a term used for a family of microscopic organisms, that are commonly cultured and fed to small fry or young invertebrates because these animals can not yet eat larger food.
Take any old dried leaves or detritus that have been lying outside (not near a road as these will be polluted), put them in a small jar of room temperature (dechlorinated or plain bottled) water and place the tub in sunlight. The eggs on the leaves will hatch out and after a day or two you will see the water turn cloudy. This is infusoria. If you have access to a 20x magnifying glass you will be able to see these microscopic creatures in a drop of the water.
They are attracted to light so shine a light down from above and you can siphon off the top layer (just under the top layer of scum) and feed them to your aquarium animals. Replace the water every couple of days.
- Infusoria naturally grows on bogwood and on the leaves of plants. So consider adding these to your tank. Some aquarists place a handful of Java moss into a small tank to raise fry as this will have infusoria in it.
These animals are tiny. Often only 0.1mm long when fully grown. So you'll need a X20 (minimum) eyeglass or magnifier to see them (you can often buy cheap magnifiers on eBay).