You can try starting the black worms in the sump first, to establish some colony, then move on some to the display, otherwise the fish will gobble all of them too quickly.
I put tons of them in a tank that housed just one platy. I watched her been as full as a baloon for months on hand. She just kept eating and eating. I now see none in there.
The ones I have in the hillstream loach tank survive under the stones. The loaches sift through the sand around the stones but can't get under them.
@
EntoStentor
I personally would not fancy the planaria idea. The one time I had planaria, I've seen fish try them and spit them out. They do not like them and do not eat them. Some species are parasitic and all of them are certainly carnivorous and can eat tiny fry and shrimp though I get that's not a problem in a "monster tank".