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These tiny critters are really not worms but rather nematodes. Nematodes are one of the most successful life forms on Earth, and there are over a thousand species of them adapted to live in everything and anything from the soil to the guts of other living creatures to oatmeal. They are extremely prolific and are slightly smaller than baby brine shrimp. It is the kind that likes oatmeal that we fish keepers are interested in here - they make a readily accepted, nutritious first food for small newborn fry. Larger fry, and even small adult fish (think Neons), and Cory cats will also eat them with gusto.
Mix up a batch of regular, unsalted oatmeal according to the directions on the box. We make ours a little thicker by adding less water than recommended and cooking an additional 15 seconds or so in the microwave. Put a 1/2" of the mixture in the bottom of a used margarine or Cool Whip tub, or any similar container that you may have, and let it cool to room temperature. The worms live on the surface, so putting more than a half-inch layer in the tub doesn't really help any. Add a spoonful of worms from an active culture as a starter. Keep the container covered to keep the flies out and the worms in (though a few small holes in the lid away from the edge a bit do help keep moisture from condensing into droplets in the tub - a paper punch works well for this). Once the oatmeal batch has reached room temp, you can add a pinch of active yeast to the top, if you want. This helps the culture take off, but isn’t really necessary to making a good culture, so it is up to you. After a few days you should notice the mixture starting to become a bit soupy. It will glisten in the light - those are the worms wiggling around. A healthy culture will have a mild, yeasty kind of smell, whether you have added yeast or not. In one week you can begin to harvest worms.
Harvesting is easy: Place the tub in a warm spot (like on top of a lit aquarium light fixture) for twenty minutes or so to get the worms crawling up the sides in good numbers - you don’t have to warm up the tub, but it helps - then use a plastic spoon, small stick or the side of your pinky to scrape the worms off the sides of the container, then swish the spoon, stick or finger in a small container of water to rinse the worms off. You can use a small dropper or pipette to suck them up and feed them to your fry. Fed from the top, they slowly sink to the bottom, but are probably best delivered just above the immediate vicinity of the fry in a tank.
Microworm cultures can last in a useful state for several weeks, but the time can and does vary. When the contents of your culture tub turns a deeper, darker brown, or the oatmeal has nearly been all consumed by the worms, simply make up another tub of oatmeal, transfer a spoonful of the soupy, glistening nematodes to the new container, and throw the old one away.