I recently introduced some California black worms to some of my tanks. They seem to be more widely used live than tubifex worms due to the latter's risk of introducing disease. So far they're doing well it seems. They've burrowed in the substrate and seem to be feeding on detritus. My tank is relatively new and has no fish in it so introduction was easy. Maybe try moving out all the fish temporarily when putting the worms in and letting the worms burrow first?