I had a goldfish (well two), cycled the tank, tests came up all clear, so brought one home it died about four days later, so changed 50% waited a week, ammonia was a little high when I found the dead goldfish, a week later tests were clear again, then over night goldfish number two died.
Can't put my finger on why, thought it may have been the tropical flakes?
Fish were healthy looking, tank they came from was good with no signs of illnesses etc.