Worms have long been understood to be an integral part of many - tropical fish's diet. While most killifish eat ants that fall from trees a few eat primarily aquatic crustaceans and worms instead, for example the Diopteron subgenus in Aphyosemion. But worms will put all killies in top condition for breeding and it's no wonder aquarium books suggest "condition for breeding by feeding worms". It really does work, nothing else seems to work as well. Fortunately they're very easy to culture, clean and disease, alage and pest free.
Blackworms replaced the Tubifex worms in the late 20th century. Two producers of California Blackworms exist that supply all of North America. They're not cultured in the sense microworms are but are instead bred in huge ponds.
The best known are African red worms, commonly sold as bait, make an excellent food for larger fish. African nightcrawlers can also be grown but are really too large for anything but large gluttonous fish.
Walter worms are almost exactly the same as microworms except they tend to remain active in the water column a little longer. Unless you're caring for bottom feeding fry they may be a better choice than microworms.