Put the temp up a notch to boost its metabolism. I alternate shrimp, smelt, and worms. One of the three eats tilapia, sure I can get em all on it if I wanted to. Within 60 seconds all food is gone, each time. Reason I hate pellets is the content of them seems to create a good chunk more algea as they dont seem to digest all the way, also increasing nitrates. Mind you this experience was with bass and not rays, but my guess is the same goes, few left over that get soft and messy dont do well in water. Just personal preference though, think we all have our "superstitions". Your ray should punce on food like it hasnt been fed in years, if it is eating leisurely I'd say something is bugging it a bit, could be normal stress from a new home as you seem to be taking the right steps. If I were you, if you really don't want to put her in the 300 right off the bat, I would remove any left over food so it gets used to "feeding time", and when its not feeding time it will burrow for food, not finding any will make it that much more prone to pounce on it when you do drop some in.
Cheers.