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In the air, there are spores, from everything from molds to microscopic animals, they are in the air and seed the water when they land in it, they then multiply and feed of the decaying organic matter and grow in size, excellent first food for fish fry, after 3 days you should see little white animals swimming around the food source, they are tiny but just visible to the naked eye.

The spores and eggs of the microscopic critters are in the natural dust all around us. When that lands in a suitable place, a puddle or a jar of decaying leaves on your window sill, they hatch out, grow and multiply.

Seeding your colony with some gunge from the bottom of your fish tank will get things started quicker.

You will be setting up a whole little ecosystem in that jar, bacteria will break down the organic material, algae will feed of the bacteria waste and tiny rotifers etc will eat the algae. It's the larger rotifers that you need to feed your fish fry.

http://en.wikipedia.org/wiki/Rotifer

If you have access to a microscope have a look at a drop af aquaruim gunge, there is a lot more critters in your tank than you may think.

Ian