Sometimes introducting a harlequin shrimp helps, but they mostly eat Asternias. It's best to just pick them out by hand. If you don't get them out and you have corals they like to hang on to polyps, making it hard for the polyps to open and they will steal food. What also might help is introducting a dottyback or a wrasse - they eat the stars like candy. Before paying for another fish though, I would ask the local fish store guy and see what he has to say - don't want to spend money and then have things not work.