Vinegar eels are harmless, white roundworms or nematodes that feed on the microorganisms commonly found in vinegar and fermented liquids. Growing up to 50 microns in diameter and 1 to 2 mm in length, they are one of the smallest and easiest live foods to culture for baby fish. Breeders commonly feed them to newborn betta fish, killifish, rainbowfish, and other fry that require miniscule foods even smaller than baby brine shrimp (which hatch out at 450 microns in size).
 
Vinegar eels have many other advantages that make them ideal for feeding fish fry. Unlike micro worms, they can survive for several days in fresh water, they swim around in the water column instead of sinking straight to the bottom, and their wiggling motions entice babies to eat more and grow faster. Vinegar eels aren’t necessarily as nutritious as baby brine shrimp (which are born with rich yolk sacs), but they’re an excellent food to feed until the fry have grown large enough to eat baby brine shrimp.