i just buy the big bags of frozen prawns from the supermarket
to get the rays to eat them cut the nightcrawlers up and soak the MPs in the nightcrawler blood over night you may want to add some blood worm into the mix as well
now in the past I have used seachems para guard in a tank that had a ray in it with no affect to the ray but that is my experience with the product now I have qtanks for these problems