Last April I was a vendor at the NEC Annual Convention. I am a hobby breeder working with the rarer B&W Hypancistrus plecos and that event is a 70 minute drive from me and the only one at which I vend. Because I am a vendor, I do not get a lot of time to attend the presentations. However, at this last one theer was one I had to. It was the breeders round table and was hel in the evening. The people on the dais were amazing. A few I knew and have fish from (one is in the same fish club as I am). The way it worked was that audience members submitted questions in advance from which were selected those asked the panel. One of the key questions was, "What is your favorite food for conditioning fish to spawn?"
The most popular answer was live black worms. But the most surprising answer to me came from a man often called a "Living Legend" in the hobby, Rosario LaCorte. He is in his 90s was likely doing his last event. His answer was Tubifex. And then came the sort of comments Colin made. Rosario acknowledge the potential risks and then explained two things. Back when he was using tubifex there were no other options. Then he explained the process used to insure the worms were perfectly safe to feed having collected them from nasty places. It was an effort but back then that edge was the key.
Today in the hobby we have black worms, red wigglers, and live BBS, white worms even cultivated infusoria and much more. So tubifex is now found either frozen or freeze dried and is safe, but not as nutritious as live.
I never worked with black worms. They smell and need refrigeration, I think. I did hatch BBS and I did use red wigglers for a while. But I found that live was one step more than I wanted to go. I now rely on frozen and Repashy foods. But I am not a master breeder, just a decent one for the limited fish with which I work..