I think the secret lies in feeding them properly. Commercially, they are fed with either gu sai or ter sai, chicken sai etc (like with boons). I fed them with the sai of my fish whenever I siphon the bottom during water change. But my experiment is only in the early stage. I just want to keep them going as long as possible so I don't have to freeze them, since a packet is a lot.
My container has shallow water about 2" depth and aerated with bare airtubing (no stone) near the water surface without disturbing the worms integrity as a clump.