Goldfish have very little nutrional value compared to other feeder type fish. I wouldn't ever feed them to ANYTHING, let alone a ray.
Rays have very specific diets in the wild, and are not 100% predatory, they do scavange a LOT. Do a little reading around this section, and you'll find volumes of solid info from the top minds in freshwater stingray keeping.
Most rays are looking for inverts (shrimp, snails, clam, worms), some are catching small fish in their diet as well.
Calcium and iodine are things I like to make sure my rays get. A well rounded diet is best. I'd suggest earthworms, market shrimp (prawn), krill, mysis (but only PE brand mysis, don't waste time with low quality junk), Mazuri Shark/Ray gel diet and possibly ghost shrimp for live food from time to time.