Vinegar eelworms

Vinegar "eelworms" (Turbatrix aceti). These tasty little nematodes are a good first food for somewhat larger fry than those tiny ones that begin with infusoria. Fry too small even for baby brine shrimp can handle these, and they couldn't be easier to culture. In fact, if you've never cultured any live foods, start with vinegar eelworms. "Vinegar eels" used to be common in home-made cider vinegar. Nowadays commercial vinegar is distilled, or pasteurized and filtered, and the discovery of even one nematode in bottled commercial vinegar would be gist for the kind of consumerist scandal embraced by early-evening network news! 
The scientific name, Turbatrix aceti, established by B.G. Peters in 1927, could hardly be improved! It means "she who roils the vinegar." You can judge how good your own culture is by holding the bottle to the sunlight. Indeed, a good roiling culture of Turbatrix shimmers in sunlight, even to the naked eye. It's hard to imagine that no one had actually observed them until Pierre Borel found them, and reported his finding in 1656, in a preface heading his enthusiastic description of the recently invented microscope: clear glass vessels weren't to be found in many European households much earlier than the mid-seventeenth century, and if your vinegar were kept in a stoneware crock, you probably wouldn't detect them either. The organism was noticed by Linnaeus and officially described by Otto Müller (1783) as Anguillula, the "little eel".
Culture. All you need for Turbatrix are a couple of clear wine bottles and cider vinegar (don't try wine vinegar or any other kind), which you dilute with de-chlorinated tapwater. If you have traces of copper in your tapwater, you may be more successful if you pass the water first through activated carbon; the acidity of the Turbatrix culture frees toxic copper ions, and the nematodes—  like all invertebrates—  are copper-sensitive. A Brita filter also contains an inhibitor of bacteria, so is not suitable in this case. 
Full-strength cider vinegar has a pH approximately 2.8-3.0. Depending on the natural buffering of your water— you need to maintain a pretty bracing acidity to keep the wrong kind of bacteria from getting into the act— you can dilute the cider vinegar up to 50%. If your tapwater is highly buffered, though, you'll do better by diluting the vinegar with a mix of your water and bottled distilled water from the supermarket.
Put a bit of peeled and grated apple or a quarter cup of "natural" unfiltered apple juice in the bottle and fill the bottle only to the shoulder, in order to maximize oxygen diffusion at the surface. You might find it helpful to add a pinch of sugar per gallon. The apple gratings sink to the bottom after a while and support acid-tolerant bacteria; some are fermenting alcohol from sugars in the apple or juice, others give off acetic acid, the characteristic acid of vinegar: acetum is Latin for vinegar, "vinegar" is a vin aigre, French for "sour wine." Above the fine white sediment that builds up, which was called "mother-of-vinegar" in the days when it used to be familiar in home-made vinegars, the Turbatrix nematodes feed on the bacteria. Keep your Turbatrix in the dark.
A culture often lasts for three months or so, then inexplicably crashes. Bill, a visitor here, had this to report: "You state that "vinegar eel" cultures last up to three months. I have one that is at least five years old, and another that is in its third year. The only maintenance that I've performed is the addition of a little more apple to the older one, and maybe I add a little water. Both still shimmer mightily."
For a back-up, I also keep two culture jars going, a useful practice for all live food cultures. If a grayish film of bacteria and yeasts forms, I insert strips of paper towel  to take it up and add a little more cider vinegar. If the culture grows cloudy or darkens notably, it's time to re-culture.
To spice a dull day, if you want Dan McMonigle's good tale of vinegar eelworms appearing in natural cider vinegar from a picturesque farmer's market see his Live-Foods Digest post.
Feeding. The only difficult trick to vinegar eelworms is separating them from the vinegar, so that you don't acidify the tank water with each feeding. Adrian Tappin at "Home of the Rainbowfish" just passes the clearest top part of the culture through a coffee filter, retaining for reuse the filtered culture solution, which will still have plenty of "vinegar eels" to carry on. He rinses the still-damp filter in clean aged water and introduces the nematodes to fish fry that way. Wayne Schmidt focuses on getting his to climb the walls of their container.  I find that a strip of household sponge, pierced at one end by a chopstick, which lies across the mouth of the culture jar, with the end of the sponge just hanging into the culture, will draw vinegar eels into it. This results from a combination of capillary action and the tendency of Turbatrix to rise towards the surface, where they expect to find higher oxygen levels. Then the sponge can be left to drain a minute, stranding zillions of vinegar eels on its surfaces. Then I trounce the sponge up and down in tank water to flush them out.
Wright Huntley's better technique, involving his "Patented Eel Sucking Machine" (looking suspiciously like a wine bottle, eh!), is presented as "Vinegar eels made easy" at the California Betta Society website.
Wytwch, in CA, first clued me in with this synopsis: "The vinegar eels grow in a bottle (preferably with a long neck) filled with a mix of vinegar, water and apple juice. When you want to harvest, just stopper the bottle down to the level of the vinegar (you do this with aquarium floss tied with string to a chopstick) and top off with water. Overnight the eels swim up through the floss and collect in the clean water at the top. They are trying to get air. Just syringe up that clean water full of eels into your fry tank. I keep two bottles going so there are always plenty of eels available for the hungry fry. I'll top off one bottle with water for several feedings. Then, when that one is getting low on eels, I just pull out the floss plug and let that one rest for a while and use the other."
She's developed a further streamlined technique: "I replaced the string attached to the floss and the chopstick with monofiliment fishing line. That way, when the chopstick is hanging down from the bottle, you don't get wicking of vinegar eel juice all over your kitchen counter as I did. Second, I keep several wine bottles full of eel solution. Put the plug in one, fill with water, let rest overnight, and just pour the eel filled water right into the fry tank. Refill with water, repeat the next day. I find I get 3 days feeding per bottle. I pull out the plug and just move it to the next bottle without rinsing, top off with water. etc... It is effortless and I have a nice column of water just full of vinegar eels every day for fry. A syringe isn't necessary if you make the floss plug nice and full; no vinegar escapes when you pour it."
Though they won't reproduce in the aquarium, vinegar eelworms will live there for days and days, so you don't have the overfeeding issues that you do with prepared feeds. They'll blow everywhere in the mildest current, but if they have half a chance, they'll always rise towards the surface, whereas microworms will sink towards the bottom. That could be a handy distinction, depending on where your particular fry prefer to feed.
Ecology of the vinegar "eelworm" culture. Sometimes yeasts and molds can form a white skin on the culture. Their normal habitat is slightly acidic fruit juices and fermented plant materials. Most bacteria do not tolerate acid; that's the secret behind preserving sauerkraut or pickles. The sphagnum mosses in an acidic peat bog scarcely decay over the passing years. Even nitrification slows down at pH values below 5.0. So the solution to eliminating unwelcome mold in your Turbatrix culture is to top up with more cider vinegar, in order to lower the pH.
Above the turbid bacterial zone among apple gratings, the vinegar should be quite clear. Acid-tolerant yeasts are working anaerobically to ferment the fruit sugars in the shredded apple, yielding alcohols and carbon dioxide and more acetic acid, which have the combined effect of keeping the pH low. All this is happening in the bottom layers of the stagnant culture, where oxygen is scarce. Most of the microbes can use oxygen if it's present, in which case they oxidize glucose and fructose completely, producing CO2 (more acidity) and some metabolic water. This process itself tends to exhaust the diminishing supplies of oxygen, so, as you can picture it yourself, the boundary layer between aerobic and anaerobic layers tends to be self-sustaining. Above the boundary layer, Turbatrix nematodes are feeding on bacteria. Though nematodes are more tolerant of very low oxygen levels than most multicellular animals, they do need some. The densest concentration of nematodes reveals where this layer persists. Look at your undisturbed vinegar eelworm culture and see whether you can't detect the various levels in this ecosystem.
"How riveting," you say, "but what's the point?" One point is that there's a somewhat similar, self-sustaining boundary layer that is normally established in the aquarium substrate, a boundary that is similarly established by facultative anaerobic bacteria— though not the acidophile ones found in this vinegar. A Winogradsky column makes a vivid demonstration of such self-organizing layering, by cultivating colored photosynthesizing bacteria in a glass column. In a healthy undisturbed aquarium substrate, only the uppermost centimeters carry oxygen in the interstitial water, among the grains. The lower levels are mostly anoxic. At the boundary between oxygenated and hypoxic/anoxic zones, facultative anaerobes keep the boundary stable, by switching between metabolisms. This stabilizing feedback between complementary metabolisms is characteristic of sulfate/sulfide interactions in the substrate, for example. This is a very useful concept to keep in mind; it will stay your hand if you're ever tempted to stir up the substrate with a gravel vacuum.