I have read that the only way to get rid of hydroids is to use a tweezer and pluck them out. If a rock is badly covered in them and is probably impossible to clean off with a tweezer am I best just to get rid of the rock? I've tried to remove as many as I could but there is just to many and some are in crevices that can't even be reached. Will they die if I leave the rock out of water for a while?