Infusoria or protozoa are useful for feeding very small fry that can’t eat bigger foods such as newly hatched brine shrimp or mosquito wrigglers.
Goldfish fry can be fed them if brine shrimp or mosquito wrigglers aren’t available.
A spawn size of say 1000 fry will require a large amount of this food for about two weeks.
By the start of week three, the fry need to be fed something more substantial as it will take a lot of cultures to satisfy their ravenous appetites.
Scald lettuce leaves or some similar vegetable matter and place it in one quart jars of aged water.
Any water that has been standing for any length of time will have a few protozoa in it to start the cultures. Don’t be tempted to use aquarium water as it will contain any number of diseases that you don’t want to expose your fry to. You will be pouring this water into your fry aquarium so be careful what source you use.
The water will turn cloudy as the rotting vegetable matter causes a bacterial bloom. The protozoa in turn eat the bacteria.
Once you see the water starting to clear, check for small white specks swimming in the culture. You can shine a flashlight through the jars at night. If the culture is thriving you will see clouds of silvery specks gliding gracefully in all directions.
The cultures should be kept at about 75oF in a well lit area. Don't put the jars in direct sunlight or the temperature will get too high and cook the protozoa.
The cultures normally take 3 days to develop the numbers to make them viable to use.
On about the 3rd day after spawning, start your cultures. The fry will take 2-3 days after hatching to become free swimming depending on water temperature.
As soon as you see some fry start to become free swimming, pour one of the cultures into the aquarium (after checking that the culture is thriving first).
Save some of the water and vegetable matter in the jar and fill it with aged water. It will take two days for the culture to replenish itself.
Observe the fry to see if their bellies are full. Keep adding more cultures until you are sure they are getting all the food they require. You will soon work out how many cultures you need.
Be very careful to check that a culture has gone through the cycle of bacterial bloom then protozoa population explosion. You don’t want to be pouring jars of water into your fry aquarium containing a bacterial bloom. The water will quickly become polluted.
Pills or powder are available from better aquarium shops for starting cultures. Just add the pills or powder to some aged water and within days you have a culture established. The pills save the hassle of having to prepare vegetable matter.