I don't know that I would try a sinking pellet as a treat for a stingray, personally. If this is a saltwater ray, I prefer treats like clams/mussels on a half shell besides shrimp, prawn and fish. Freshwater rays would probably enjoy the molusks as well but I'm not sure how they'd be recommended in terms of nutritional value for freshwater. For motoros, teacups and leopoldis, I like worms & live shrimp. Iff you should offer a pellet, maybe try the sinking giant carnivore pellets by hikari, but imo I think a variety of live&frozen foods are best for such exotic fish.