Just read the thread for ya, a couple of quick thoughts from me:
Pups can get stressed from bigger rays, and that could be part of the problem.
About the 'internal parasites' - where the rays wild caught or in with wild caught? Trying to figure out the source of the parasites. I wouldn't treat until I knew what I was going after. If they are wild caught and you figure out Prazi is your best bet, make sure to keep the oxygen levels high for them. I'd skip salt. To me it's a 'make the owner feel good' treatment, as it's never been proven to do anything. Salt changes the ability of parasites to osmoregulate themselves - often time having them fall off... but it does the same to the rays. You're impacting the ability of the rays to keep their bodies in equalibrium with the water they are in. Not an issue for a healthy ray, but it can add stress to a sick ray.