He is just having trouble catching them. I put a crayfish in and he nipped him a couple of times, but hasn't killed him yet. The crayfish hides behind a plant in the corner.
He eats his bloodworms ok. He has also eaten a couple of guppies. I think it is just immaturity. I put some mystery snails in there, but he doesn't seem to know that it's food. I guess he will learn.