Daphnia

Daphnia and Bosmina and Moina species are all "Daphnids". They are classic live foods for fish. Daphnids are sometimes called "water fleas" because of the jerky, hopping movements in the water made by the most familiar species. Not all hop, though; others scramble over leaf surfaces and detritus. The largest species can get bigger than 5mm, but many are less than a millimeter across.
 
The daphnid genus Moina, the one more suited to indoor culture on a small scale by a fumbler like me, can be thought of as a "mini-Daphnia" that makes a good substitute for Artemia nauplii (brine shrimp).
 
Daphnia, Bosmina and Moina are cladocerans, primitive freshwater crustacea found in green eutrophic ponds and temporary waters, more common and richer in species in temperate waters than in the tropics, but eagerly taken by freshwater tropical fishes. Daphnids are associated with more or less alkaline water and found wherever the waters aren't home to fish, which tend to hunt down and eliminate cladocera. Water with low calcium levels and a pH below 7.0 tends to favor copepods ("cyclops") which devour the smallest daphnids and take over the culture.  If you are culturing indoors, the daphnid you're after is Moina.
 
In unpredictable population blooms that occur when conditions are right, sometimes temperate-zone Daphnia can be dense enough to temporarily color the water. In boom times, Daphnia populations are all female and produce female offspring from unfertilized eggs, a trick that's more widespread in the animal kingdom than you might think. All the viviparous offspring are identical twins of their mothers, with no genetic variation. When times get rough, as the weather starts to cool for example, males begin to be produced, and daphnids begin to carry thicker-skinned fertilized eggs, which remain behind in the cast-off carapace when they moult. The overwintering egg-case is called an ephippium, because its folded shape reminded an early observer of a saddle-cloth, called "ephippium" in classical Greek. The eggs preserved in this life-style technique that is called diapause represent the sexually-produced generation; they embody the genetic variation essential to continued adaptability in a species.
 
Hunting daphnia with long-handled fine-mesh nets used to be a classic occasion for bonding with the other members of your fish club — and coincidentally for introducing all kinds of interesting predatory insect larvae into your aquarium. Nowadays organically rich but unpolluted village ponds and farm ponds that teem with Daphnia in spring and fall are mostly a memory.
 
The Daphne for whom Daphnia are named was a mountain nymph of Thessaly in northern Greece, a pre-Hellene priestess of the Earth Mother. The aggressively patriarchal Greeks tried to invent for her a "father"— the River Peneius in Thessaly. The invaders' god Apollo pursued her to possess her, a mythic formula for a historic event that was more social and political than spiritual. But Daphne called out to the Mother Goddess, who changed her into a laurel tree. The crestfallen Apollo had to satisfy himself by wearing his familiar wreath fashioned of laurel leaves. What's the connection? Why am I telling you stuff like this? The scientist who named the species Daphnia must have known some Renaissance painting representing the myth, with the lovely nymph's outstretched arms sprouting leafy twigs, such as Antonio del Pollaiuolo's! Will you ever see Daphnia quite the same again?
 
If you put some daphnia into fry grow-out tanks, they'll consume uneaten fry food and protozoans that might degrade water quality; then, as the fry grow, they'll eat the daphnia.
 
Culture. Daphnia can be cultured outdoors or on a south-facing screen porch, in tubs of green water. Daphnia, and most cladocerans, require high levels of dissolved oxygen; they tend to suffocate at high summer temperatures or when organic decay processes use up too much of the available oxygen. So keep Daphnia cool and well-fed. If you are having mysterious trouble culturing Daphnia, try checking the copper levels in your water. Even quite low levels of copper may be stressful to daphnids, and if the pH slips, those same levels of copper can become lethal.
 
Culturing Moina. Indoors, the miniature Daphnids, Moina species, might be a better choice. Moina species average 1.2 to 1.6 mm, whereas D. magna can get up to 5 mm. Moina survive warmer temperatures and lower oxygen, and they're small enough to substitute for newly-hatched brine shrimp nauplii in your fry-raising schemes. If you're culturing green water, you already have the basic requirement for culturing Daphnia and Moina. If your water is soft and acidic, you'll need a layer of crushed coral for a substrate, to keep the pH above 7.0. Lower pH levels discourage daphnia and encourage their copepod rivals. Neglect a culture, and you may find it's been overwhelmed by copepods, which thrive in similar set-ups but lower pH.  
 
McDaphnia posted these suggestions about culturing Moina at AquariaCentral, March 2002: "Moina are like Daphnia only smaller. The newest, smallest ones are about twice the size of an adult rotifer, so they are an excellent first food even for fry too small to eat baby brine shrimp. Some breeders believe they get better growth and survival rates when feeding Moina than with feeding baby brine shrimp. Once you have a few Moina eggs, you should never have to buy more. They reproduce more easily than Daphnia. If you make a mistake and kill a culture, just let it sit, and in a few days to a couple of weeks, it will reset itself and start over, once the problem, which was usually over-feeding, has had time to correct itaself. The eggs are very hardy and protected in a flat black case. Eggs will hatch numerous times, so they can be used over again for years anytime your Moina culture needs revival. Each Moina egg is really a packet of eggs called an Ephippium. It is saddle-shaped and rests on the female's back where a saddle would fit. Each time an ephippium is exposed to a wet/dry cycle, only a percentage of eggs actually hatch. This protects the Moina from being killed off in the wild by an early rainfall that dries up too soon. This survival mechanism also works in the fish room and makes Moina-keeping almost foolproof.
 
"It seems easier to start a Moina culture from eggs, because your water is the only water they know. Live Moina or live Daphnia have to adapt from the water they were raised in to yours, which may be different in its chemical composition, even if it measures the same hardness and pH. However, it will take a couple weeks longer to reach production levels where you can start regularly harvesting these for your fish, when you start with Moina eggs.
 
"The eggs can be shipped dry, folded into the corner of a piece of paper. They will look like a dash of black pepper. Just drop them in water when they arrive. One way to do this is to add them to a small bare aquarium, no filter, no gravel, air optional. Once the Moina start to hatch, siphon out the Moina and all but a fraction of an inch of water. Begin culturing the Moina you collected in a larger container, such as a five or ten-gallon tank. Meantime, allow that fraction of an inch of water to evaporate naturally. The eggs can be scraped off and stored, or just left in the small tank for future use. If you should take a vacation or just not have any fry for a while, stop feeding the Moina cultures. They will reach a low, stable but naturally pulsing population and stay there indefinitely, until you start feeding them. A pulse is an increase in the population, followed by a low population that then repeats."
 
Moina in the soft conditions of captivity never produce males, only parthogenetically produced females, each an exact genetic copy of its mother. McDaphnia also has a technique for "wintering" a culture: "Putting some outside in the fall and letting them freeze solid seems to do it. In the spring when the block of ice thaws, the Moina start swimming and are soon coated with "saddlebags" of eggs, which they shed as they molt. Subsequently they give live birth and never produce more eggs. It will be their distant descendants subjected to freezing conditions a year from now that produce eggs next time."
 
Culture: "Fill a 10 to 20 gallon tank with hard alkaline medication-free water. Drop in the paper square with the ephippia. Once they begin to hatch, scoop or siphon most of the water and hatchlings off into another similar tank to be cultured and fed, leaving the eggs and a fraction of an inch of water and the unhatched eggs in the bottom. To protect the species from false springs and short wet seasons, only a few eggs hatch with each exposure to inundation. Let the eggs dry out in the nearly empty tank, wipe them up with a bit of paper and store them until the time your current Moina culture is lost or needs 'new blood'. Moina can be cultured in purely fresh water or in brackish water up to half the salinity of seawater. If you have soft water, I would add about 10- 20% saltwater. Water-change water from a reef tank can be reused for this. Foods and care for Moina is the same as for Daphnia, but Moina are more productive, more dependable, and less prone to crashing or die off."

I couldn't improve on this. My own moina thrive on a teaspoon of flour, a pinch of dry yeast and a teaspoon of non-fat skim milk shaken up with some water from the plant nursery and added sparingly, just enough to cloud the water.  A dry pea gets dropped in now and then and some algae-covered Java Moss is left to break down. In a few days, when the water's cleared, moina are scooped up in a small cup where they gather near the surface under light and fed to fish.
 
From the time the egg hatches to the first release of young, under optimal conditions, is 4 to 7 days, according to the University of Florida IFAS document linked further down.
 
Culturing links. Check out Kai Schumann's "Daphnia FAQ" from 1996, now archived at Reef Traders' Post. It's a splendid information resource.
 
For your second Daphnia resource, read John Clare's page "Daphnia: an aquarist's guide"  to Daphnia and their culture that filters and weighs some conflicting advice on rearing dapnids and gives lots of links.
 
Killifish keepers depend on daphnids. Jim Langhammer, the retired curator of Belle Isle Aquarium gave rather meticulous techniques for culturing D. magna, the strain of "Giant Russian" Daphnia that has been passed round among killi enthusiasts for the last forty years: it's at the International Killifish Association site. And Doug Sweet's "Daphnia made simple" is at the Minnesotas Killie Keepers Association website. 
 
Moina biology and ecology and complete instructions for reviving Moina eggs and feeding the Moina culture are given in a long article that Gay Hemsath posted in Sept 1999 to the Native Fish Conservancy mailing-list, archived at FINS .
 
McDaphnia's "Daphnia Cookbook" offers tasty recipes for Daphnia, not of Daphnia, all served up with the usual witty McMonigle sauce.
 
Finally, the University of Florida IFAS Extension site weighs in with a thorough guide to large-scale, "Culture techniques of Moina: the ideal Daphnia for feeding freshwater fish fry," which is more especially suited to aquaculture, however.
 
Increasing population density and urbanization in the island-republic of Singapore have motivated techniques for highly efficient pond culture of Moina micrura for Singapore's aquarium fish farms, using wastewater from pig farming. You'd think that these large-scale daphnid culturing techniques wouldn't be very relevant to your aquarium concerns, but I noticed in a document now removed from the University of Singapore website that they drain the meter-deep ponds and let them bake dry in the sun every three months or so, to eliminate competing organisms like hydra and copepods and  ostracods, which have no drought-weathering stages.