Successfully raising very small fry from egg scattering species requires equally small first foods. Many club members keep active cultures of vinegar eels and microworms, or hatch Artemia nauplii, usually referred to as Baby Brine Shrimp (BBS) to feed the small fry. This works well for species having fry large enough upon hatching or fast enough growing to take BBS either immediately or within just a few days of hatching. However, these nutrition sources are generally inadequate for raising very small fry that may not be big enough to take BBS for many weeks. Peacock gudgeons and drape fin barbs are good examples of larval fish that take several weeks to reach a size where they can fit BBS in their tiny mouths.
After much experimentation and many failures, I've settled on the following feeding progression in order of increasing size for very small fry:
Biofilm (slimy oak and magnolia leaves, java moss, driftwood film)
Vinegar eels (for surface dwellers) and microworms (for bottom dwellers)
Blepharisma (another single-celled ciliate like Paramecium)
Brine shrimp nauplii
In an ideal world, I would also have green water to go with the biofilm for just-hatched fry, and Moina macrocopa to go after the Blepharisma, but sometimes it already feels like I spend more time raising food than fish, so this is what I do. Growing Moina or Daphnia requires frequent, regular feeding which doesn't work with my schedule. Infusoria cultures, on the other hand, are simply fed boiled wheat berries every few days and virtually maintain themselves for long periods of time until the culture is eventually contaminated by something that eats the culture, usually some tiny worm. When this happens it's immediately apparent because the water turns from cloudy to crystal clear.
Pure cultures of euglena, rotifers, paramecium, blepharisma, moina, and other infusoria can be sourced from Carolina Biological Supply. Or, you can grow mixed, impure cultures just by soaking some rabbit feed in jars of water (or one of many other approaches). I like to keep the pure cultures going so I can time the size fractions and because you can get some predatory species of infusoria when using a more uncontrolled approach.
The euglena, rotifer, paramecium, and blepharisma cultures are maintained in 2 liter Nalgene collecting bottles with a hole drilled in the top just large enough to accommodate a rigid airline tube. Provide gentle air at about 1-3 bubbles per second. Add a small amount of crushed coral to buffer the pH (a small pinch of Baking Soda can do the trick until you buy a bag of coral at the LFS). Boil some wheat berries (rice grains will suffice for a few days until you get to the grocery store to buy wheat) for 10 minutes to sterilize and soften them. Feed 1-2 wheat berries to each culture every few days. I usually just boil up a hand full, put them in a small dish next to the cultures, and use them for a week or two until they run out. However, in this case you need to be very careful not to cross-contaminate your cultures by sticking wet hands that were handling one culture in the dish to fish out wheat berries for the next culture. This is why I always proceed from the smallest size fraction culture to the largest when harvesting or feeding. If I inadvertently introduce euglena into the rotifers, or rotifers into the paramecium, or paramecium into the blepharisma, no big deal, the larger organisms will probably enjoy a snack. However, introducing blepharisma into the smaller-sized organism cultures means another order into Carolina Biological Supply to start over.
To harvest the cultures without fouling the fry hatching and grow-out tank water, it's best to use a 44 micron plankton sieve. I also have a set of sieves ordered from www.aquaculturenurseryfarms.com. I use my sieves nearly every day. First I pass skanky infusoria water through the 250micron to remove large chunks of bacteria-laden wheat berry, and then through the 53micron to collect the food organisms. The food organisms are then rinsed into a cup or beaker and fed to the fry. Most people don't bother with the sieves. They use a flashlight to get the infusoria to congregate and then remove them with a baster or pipette.
I can't see euglena or rotifers, even with a magnifying glass, but I can see the paramecium and blepharisma with just reading glasses. The only reason I'm confident that the euglena and rotifer cultures are good is by watching the reaction of the fry. If they go nuts and start darting all over the tank after the addition, I assume everything is good.
While it's always gratifying to see fry feasting on rotifers or BBS, these organisms move through the water in a rather comical, lumbering fashion and don't present much of a challenge to the fry. There's nothing quite like adding paramecium and watching the fry immediately "hunch down" into hunting mode. Fry quickly begin to develop hunting strategies, often assuming a vertical position, either head up or head down, against the wall of the tank and remaining motionless until a paramecium comes into striking range. It's fascinating behavior. Nothing seems to set a little larval fish's heart aflutter quite like paramecium.
Anything Frank Cowherd says
Anything John Mangan says
Culturing Live Foods by Michael Hellweg
David Ramsey's website, David's Live Fish Foods (be careful when navigating, website isn't secure and malicious third party pages may open in a separate window)
David Ramsey's YouTube channel
Carolina Biological Supply Living Organism Care Guides (https://www.carolina.com/teacher-resources/living-organism-care-information/10849.com)