If you want to add variety in opus's diet, you can try gutloaded crickets, earthworms, snails, guppies. Just dont ever feed opus anything like 'crab, crayfish, etc' like tenodera said, they can harm your turtle, if not get him sick..and I know you dont want that, right?
Just remember that all of the foods I mentioned above shouldnt be fed too often, i'd say once every two weeks is fine. Try adding an aquatic plant in there! They are very nutritious for opus, it offers him a snack whenever he wants and will help keep the water cleaner as well!