I concur with post #13, but I would consider metronidazole in the food before I would start dumping "medications" in the tank. Twice I had mysterious die-offs (after adding new fish, lesson learned about a quarantine!) and following the advice of a marine biologist I knew on another forum used metronidazole. She said it is much more effective if taken with food, so I prepared flake and pellet food and fed it exclusively for ten days. Both times the fish deaths did stop. She suspected some sort of internal protozoan which would only be identified with a necropsy, by which I mean no external sympstoms other than a fish suddenly having trouble swimming and dying quickly.