Are you looking into buying one or have you acquired already?
I’m no expert here by any means, but I am pretty serious about my fish.
Typically rays eat a carnivorous diet, worms, insect larvae, small fish. It is highly recommended to get them on a good pellet with a decent amount of protein. I’d never recommend feeders especially gold fish. Some may only take live foods if wc, but you don’t want to do this long term if possible. I feed mine 3 different types of pellets, small chopped frozen tilapia(thawed), market shrimp, red wigglers, brine shrimp, blood worms, and I usually supplement a few times a week with probiotics in these foods.
Rays need pristine water because they are super sensitive to ammonia. They also create tons of waste so it’s easy to let it get out of control. Which means lots of maintenance. I do a 50-70% wc 2 times a week. I also do a spot clean almost daily from all the big chunks of poo the filters don’t catch, so replacing 5-10 gal of water each day. Lots of work- but man I don’t even care it’s worth it. Because of the frequent wc - you want the rays to be adjusted to whatever your ph is in your tap water. Not sure off hand how to do that myself, but I’m sure you can find that in here somewhere. Lol.
Once again I’m no expert so some of the OG’s might have some better advice. Lol.