they cant catch them all, esp if you have gravel substrate for them to hide in, i have permanent cultures of them in some tanks. beware of sponge and other filters cause they will set up shop in there and then cant be accessed for food, although they will be doing a sort of biological filtration. the problem with black worm cultures to newts health arises when and if the cultures crash, the bio load is huge for a large mass of decaying worms and this can easily poison a tank and kill some of the newts. for my tanks with established cultures of them i have some sections with gravel (like where plants are anchored) and bare sections in between, the worms move into the gravel sections and spread out like grass-but its smart grass and often avoids the newts-so i go in occasionally and stir up the gravel so that the worms are everywhere and vulnerable to predation (im soooooo mean!). on bare surfaces they have habits of forming into balls and then it takes a while for them to sort themselves out and hide back in the gravel, in the mean time many have been eaten. newts can however eat to excess on bare bottom empty tanks since they are just giant balls of worms and too easy to over consume - i have seen juvies with them crawling out of their gills as well as adults so full that they crawl out of their nostrils! even with bare bottom tanks plants such as elodea allow for the worms to disperse to lesser, more stable, densities.
