Never had a problem with mine. He is about 6 inches long. I hardly ever see him though. Maybe once or twic a month I will see him reach out to grab a piece of krill. Really not worth having if you want to see them. Hope this helps.
Mine has never harmed a fish or my pistol shrimp, but he is not with any zoo's or SPS. The LPS and lethers are not near the bottem so he will never get near them. He is about 2 years old and only comes our when I feed and never comes out very far.