Daphnia is a good food for small fish and fish fry. While the daphnia sounds like the name for a hot chick, it is actually looks and swims in "hops" like a flea such that they are also called water fleas even though they are not fleas at all.
Daphnia can be cultured and harvested using green water or liquid fry food although they love eating bacteria. However, unless you are a commercial fish breeder it is not worth the effort as they can be bought cheaply.
Live daphnia come in bags that look like greyish cloudy water. If you look closely, you will see that it is made up of a great number of flea like stuff swimming in hopping motion. If you should also see some clumps of blackish solids in the bag, siphon them out and discard. It is chicken poo, which comes laden with bacteria, which is what farms use to feed daphnia. Don't worry about the water having bacteria, the daphnia will take care of that.
To use the daphnia, use a fine net (those really fine ones that looks like panty hose material) and give it a few scopes in it the daphnia bad. After that, rinse under slow running water and they are ready for feeding!
Note: When you introduce daphnia to your tank you will also introduce some bloodworm eggs into the tank. If you have a bare bottom (no gravel/sand) tank, you will soon find longish brown stuff at the bottom. These are cocoons with blood worm growing inside. These can be left alone because blood worms are harmless to fish and fry. (They don't suck blood! their name come from their red colour)
Tip: If you have cloudy water from overfeeding your fish, daphnia will clear it up! (But make sure you have small fish that eats daphnia because big fish ignores them).