More Info About Black Worms
Thank you, lelei. *hugs back*
So here are a few things I've learned about black worms.
According to The Betta Handbook by Robert J Goldstein, they are found in raceways at trout farms and in the effluent treatment ponds of vegetable processors in California. I don't know if they are found outside of California or not. They can't live for long in their own pollutants so if you plan on feeding black worms, you must rinse them in cold, dechlorinated water each day. If the water doesn't run clear or they don't tangle into a ball, throw them out.
Like tubifex worms, which are found in sewers and wastewater, black worms have a high risk of transmitting parasites to your fish. A good way to avoid this is to rinse the worms as noted. This purges them.
I would highly suggest cutting the worms into small pieces to avoid the possibility of them surviving ingestion and tearing the stomach of your betta. Feed only 1-2 worms at a time to avoid the engorging that happened to Train (little glutton snatched 4-6 of them off the net while I was trying to get the worms off).