Rotifers

Rotifers form a whole phylum of mostly microscopic organisms that are a major element in your "infusoria" culture, as well as in the natural freshwater plankton.
 
Cultivating a dense population of rotifers is beyond my capabilities, but in the wild they form an important prey item for fish fry and a first food for larval fish in captivity. The nourishment value rotifers offer largely depends on what they've been eating, which is concentrated in their gut. The rotifers in a mixed "infusoria" culture are going to be largely the free-swimming ones, but many rotifers can fix themselves with a sticky foot to solid surfaces, either to flotsam and jetsam in the water or to the biofilm; small fishes picking selectively at leaves in the aquarium are plucking up rotifers and ciliates.
 
So, though purposefully culturing freshwater rotifers as fry food isn't so common,  they'll be present in any infusoria cultured from water from a planted tank. There are only a few species of marine  rotifers, so they don't form part of the usual marine plankton, but in commercial marine  aquaculture, culturing Brachionus species is standard procedure for raising the larvae of  fish and crustacea. In freshwater Brachionus rubens and B. calyciflorum are the ones most generally used. You should check out the UN's FAO utterly technical and practical Manual on the production and use of live foods for aquaculture  section for culturing Brachionus plicatilis rotifers, grown on yeast or marine algae.
 
McDaphnia posted to the live-foods digest, 31 Mar 2000, "The marine rotifer, Brachionis plicatilis, is suitable for feeding to both marine and some freshwater fish. I have fed them to adults and fry of an albino strain of Julidochromis ornatus, an African cichlid, and any other small African cichlid fry that there were still excess rotifers for. The albino "Julie" fry had difficulty eating brine shrimp, and so only a few fry would survive from each clutch of eggs without the rotifers."
 
Brachionis rubens and B. calyciflorus are freshwater species and so would be more logical to use with freshwater fry, but if you find it easier to culture the complex of rotifer species grouped as  B. plicatilis, try it with some freshwater fish, especially those from hard alkaline waters.
 
The Plankton Culture Manual, by Frank H. Hoff and Terry Snell (available from their Florida Aqua Farms) covers the freshwater species B. rubens and B. calcyflorus and the marine species B. rotundiformis and B. plicatilis.
With the marine aquarist in view, Dr Frank Morini explains "Rotifer home culture" in brackish salinities at the Advanced Aquarist site.