So, I started purchasing crappies from the local bait shop. They are 12 for a dollar and when I ask for five or more dozen ($5 or more), they give me a whole bucket full of crappies. I usually freeze half (In a large ziplock freezer bag placed flat so they make a sheet of frozen fish), and put the other half in my feeder tank (cold water tank) and feed them high quality fish food.
Consequently, my rays (and my other predatory fish) have learned to eat both the frozen (kinda like silversides) and the live crappie minnows (which have been enriched by eating the high quality fish food). Its super cheap to feed this way and you weed out them possibly carrying a disease by freezing them, or quarantining them wherein the unhealthy ones die within a day or two while awaiting their fate in the feeder tank, and the process of freezing kills off possible disease or parasites.
Now, I still feed them a variety of food including: shrimp, night crawlers, bloodworms and blackworms (I usually mix the two to stretch the blackworms), raw fish slices, and pellets. But the crappies make the food stretch for a long time. And my rays started growing like weeds...waayyyy faster than before!