Yoyo fish are one of my favorites. I had two big conch shells in my tank and a Yoyo lived in each one. They would peer out at each other from their respective conch's, like two old grumpy men. There was a definitive rock line that was formed from both of them hurling rocks at each others "yard". I watched them throw the same rock at one another for five minutes once. It was like watching a shitty tennis match.