I would suggest NLS hexshield for hex or hith. It's a medicated food, so unless the rays eat small pellets (small hard not tasty pellets) they likely won't have a problem. I've also read that treatment is much more efficient when absorbed through the gut but
RD. Could probably tell you more. To answer your question however, I am not sure.
Needs to be a water treatment as the discus effected aren't eating and must be Ray safe as I have a Ray in there , the Ray also eats anything including small pellets