Sometimes your fish may fight in the tank. It can be a territorial issue, or perhaps the fish are the same sex. Some fish don't like it when the encounter a fish of the same shape (this occurs with tangs often) Above, you can see two different Target Mandarins fighting. I didn't expect there to be a problem in a 280g reef, but they just didn't like each other. Oddly enough, after 5 minutes of this tugging action, they would switch positions and the other did the exact same thing. Here's a 2 meg video. Sometimes they work things out, sometimes it leads to death.