Get rid of the damsels. The yellowtail can be very aggresive even when its small about an inch, inch and a half. When I was getting started in the hobby I had added a koran angel and within 4 days it was dead. I thought that it might have been my water or something else i was doing, but everything was fine. I returned my dead koran to the store where I got him and got another koran. Second day in the tank he was up on top and clearly stressed. Third day he was on the bottom, in a defensive position and very much stressed. He was very washed out looking and he had small cuts( just like my first angel). Again I thought it was something I was doing. Fourth day, I just happened to walk up to the tank while the lights were off. Both of my yellowtails were attacking the koran and nipping at him. A few moments later he was a floater. I got rid of all my damsels and have never had another problem.
