What could kill a fish other than water problems or temperature?

Recently my little clown fish disappeared. I had a pair, a large one about 5 years old and little one about 2 or 3 years old. I've had them for two years and they've been through a lot, including a three hour move to a new home. I have excellent filtration and tested all of my water levels and they are perfect. Recent additions to the tank have been a peppermint shrimp, serpent star and a royal gamma. I haven't seen the royal gamma picking on my little fish. I'm uming my little clown has died but I have no idea of what! The only major changes in the tank other than the new additions was that my last light fixture blew out so I had to order a new one and they were without a light for about 2 weeks. They've been longer than that without light when they were being kept by my dad until I could move them here and he didn't take care of them very well. They survived that, what could have killed him? There has been no signs of disease on any of my fish and the rest seem to be doing just fine.