I had the same problem. Two things worked for me great. First, I stopped cutting up the nightcrawlers and made them work for their food a little more. This helps them strengthen up their jaws and makes them more aggressive feeders because they have to fight their food more to subdue it. Secondly, I fed them a ton three times a day until they can't eat anymore. them starve them for one day and throw shrimp in no shell and cut into small chunks a little bigger than they can eat easily. this worked like a charm. within a week they were mauling shrimp.
Another thing you can do is get another ray from someone that is already trained over to show them, or other fish to compete. I have a school of tinfoil barbs which help to make everyone fight for food a little more, and I haven't seen a ray terd since they were introduced. They eat them as soon as they come out.
Another thing you can do is get another ray from someone that is already trained over to show them, or other fish to compete. I have a school of tinfoil barbs which help to make everyone fight for food a little more, and I haven't seen a ray terd since they were introduced. They eat them as soon as they come out.