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Vinegar eels are an aquatic nematode that are not eels, but do live in vinegar. They are a couple of millimeters long and are very easily cultured.

Take a one gallon glass jar, and add 5 parts water and 1 part cider vinegar. Add a small piece (one eighth) of an apple and inoculate with a starter culture. In a couple of weeks or so, you'll have a wriggling mass of "vinegar eels". They don't seem to care much about temperature 20 C / 70 F plus or minus quite a few degrees either way is fine.

To feed them to fish, siphon them out into a net that you've lines with filter paper or a paper coffee filter. Siphon a bit of fresh water over them once or twice to remove all the vinegar and dip the paper into the tank.

Although primary used to feed fry, many small killifish (Diapterons, etc.) can eat them their entire life.

You'll need to be careful as you'll gather hundreds of not thousands when you siphon them out, and you don't want to overfeed. They'll live for a while, but not forever, in fresh water.