To obtain infusoria, you take a handful of hay or dry leaves and place it into a jar, fill the jar with water from your tank or a pool of water. Leave in the sun for a few days. When the water just starts to get cloudy large unicellulars have sufficient bacteria to feed upon. Once the water begins to clear again, an appropriate culture is available. You can then extract these creatures, with a turkey baster, for example, for feeding to your fry.
To keep infusoria on a continual basis at home, use a mixture of at least two to three of these ingredients: hay, dry leaves, grass blades, vegetable leaves, stalks, or cucumber skin. Add water to let them ferment in a plastic container (tub/tote) that is somewhat flat and provides a larger surface for oxygen to enter the water than a jar. Expose to indirect sunlight. Depending on room temperature, bacteria will thrive during the first three to five days (faster in warmer temperature). The fermenting liquid will give out an unpleasant odor and cloud the water. This liquid should not be fed to fish fry because it contains a high concentration of amonia (or acid if your raw materials contains too much starch) and the bacteria it contains are too small for the fry.
After the initial three to five days, pour 1/3 of the water into a second, smaller, flat, container. Do this twice a day, because fish fry will benefit from frequent but small helpings. Top up the tub with non-chlorinated water (tap water left to stand overnight will do). Add fresh leaves or vegetable scraps every three days. Harvesting should be done regularly, such as on a daily basis, to prevent overcrowding of microorganism that can result in the collapse of the culture.
Dilute the fermented liquid in the second container with another 30% to 50% non-chlorinated water. The liquid at this stage will still smell earthened and unpleasant, but not pungent with amonia. Unicellulars should bloom and peak within 12 to 24 hours. Some of these single cell creatures may be visible as small, mobile, dots by the naked eyes against a dark background and if bright light is shone from the side, which makes an interesting science project. For example, you can place a clear plastic container on black paper upon window sill, then wait a few minutes for any debris to settle before viewing. The microorganism may congregate near the water surface for oxygen and you need to use only the top 2/3 of the water. A kitchen sieve should catch any debris while allowing the unicellulars to drain into your fry nursery tank. The tank should be weakly aerated to prevent algae scum feeding on the nutrient-rich water from accumulating at the water surface and blocking off oxygen.