The free-living nematode Panagrellus redivivus (sour paste nematode) is known to many aquarium enthusiasts and fishkeepers as the Microworm. It is a tiny roundworm used as the first food for minuscule newly hatched fish, such as the popular pet fish betta, killifish, etc. Microworm is widely used in aquaculture as food for a variety of fish and crustacean species. Microworm is about 50 micrometers in diameter and approximately 3 mm in length, barely visible to the naked eye. It is easily cultured at home on a substrate of flour or porridge. Microworms are easy to see, you can see shimmering on the surface of the culture media.