Good Idea! Fresh food is going to have the highest amount of nutrition. Also, being a freshwater fish, it will be more digestible and healthier for them.
You could fillet the tilapia right out of the water and drop it into the ray tank.. also harvesting the eggs from females would be an interesting meal.. I also wonder what kind of internal organs rays would eat and/or would be beneficial to them?!
Keep up the good work! Maybe Nic will chime in