Like the video shows, it will work when the rays are young, but when they get older, the rays become an issue. I have had a 17" stingray catch 8-10" tinfoil barbs, scale them and then proceed to take their heads off. No exaggeration. And tinfoil barbs are pretty robust fish, especially when compared to discus. I have seen it work with larger rays, but when they get hungry, they can eat fish. Discus can make nutritious, but very expensive meals.