I've had two yellowtail damsels in my 200L tank for some time now, along with a small clownfish. Everything was fine until yesterday I added a medium clownfish (bigger than the two damsels).
At first they chased the big one for a few minutes until my fire shrimp came over and gave him a little 'cleaning' and then they backed off. The two pairs stayed on opposite ends of the tank. Everything was fine when I went to bed but when I woke up I find they small clownfish missing his two bottom fins and the one on his side in pieces. 😢 I've isolated him and he is struggling to swim but not too badly. He occasionally lies on the bottom but gets back up seconds later. But this is the one the damsels have been living with for ages. Why attack him and not the new guy? Is there any way I can calm down the damsels? Feed them more? I know they can be aggressive at times but I read from several places and heard from my LFS
that my tank was big enough to house the pairs without trouble.