Many years ago (50+?), we used to get Tubifex worms, probably from the sewer ditches of Mexico. The (mostly imagined) introduced disease problem got them banned in CA. Now Novalek brings them from E. Europe, and purges them well before packing in the mini breather bags.
Tubifex can have real junk in their gut, but a few days in clear running water purges all that as well as most surface bacteria. I observed that in the earliest days of Discus breeding, when one of the first pairs to breed in the US was housed at the Altadena Water Gardens in Pasadena CA. Those Discus were worth thousands of dollars, and were regularly fed copious amounts of Tubifex worms.
Even after purging, Tubifex and Blackworms will quickly kill your fish, but not for the reasons you think. They are very rich sources of nitrogen, and their wastes are super high in ammonium, At high pH that can turn to deadly ammonia, and at lower pH the bacteria can quickly convert it to nitrite which displaces oxygen in the blood and causes "brown-blood" disease.
Salt can help the latter problem if your water isn't too soft. For either problem, the water needs to be cleaned by more frequent water changes and lush plant growth when feeding rich foods like worms.
We had highly chlorinated water when I was a student in So. CA. I just put an ounce or two of Tubifex in a quart canning jar and submerged it in the tank behind the toilet. It thus got several water changes a day, flushing out any dead worms and debris, but those were normally not visible in the toilet. It worked great and the worms never introduced any outside pathogens that I could identify. [Heavy feeding to condition breeders did give me the bad results above when I neglected a water change.]
Don't be afraid to feed worms, as their other byproduct is greatly improved egg quality and quantity. Just be aware that they are rich so you need to react accordingly.
It has cooled off, here, so I just ordered another pound of blackworms from Fresno. Blackworms have much less hemoglobin, so I cannot keep them in the toilet tank without drowning them. :-( I need to clear some refrigerator space for them.
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