I know small trace amounts of copper kill those lil white worms. I dont have any but i was goin to put dome in for added saftey. I have some in my other tanks but then again they dont have rays in them.
Honestly I would just raise the salinity of the water. I salt kills a lot of the parasites, and the rays should be able to handle a low salinity of around 1.002 or 1.003.