most likely it had an internal bacterial infection, and this may have caused loss of apitite, the main thing now to worry about is the remaining fish, I personally would do a couple of extra water changes this week, and also add a little aquarium salt (1 tablespoon per 5 gallons) also keep a close eye on them B)Sasha said:The pH went up a little bit, but I managed to correct it. Besides the pH, everything else was fine. My tankmates include guppies, mollies and tetras, so I don't think they could have harmed her. I examined her body many times, but I found no signs of disease or any marks. She was a little thin though. Is it possible she could have starved herself?