I have a MASSIVE one. It is harmless, other than knocking over my frag stands. It eats detritus and stirs the sand better than anything you can buy to stir the sand. They do get HUGE! Mine, when last seen, was 6-7" shrunk up and well over 12" stretched out. Absolutely beatiful when they get big enough for the red and white to be bright. I've read a few places not to let the bristles touch you. They are supposed to be very painful.