Tabu,
Alot of research has been done in the wild in regards to the rays natural diet.
80% of their diet in the wild consists of some form of crustation.
Try baby crayfish also. Shrimp is great has alot of zinc.
Earthworms are great food, I have never known a ray to refuse them.
Try smaller red worms ( europeon crawlers) they are smaller in size.
HTH.