Fish is really happy and eating one day, dead the next

Goldfish are very fragile and slight changes in some parameters in their environment results in death. For example, they have zero tolerance to ammonia and nitrites and are poisoned by them. Other causes include oxygen deprivation associated with low pH, supersaturated gases, enclosed tank and lack of proper water circulation. Nitrate poisoning may also have caused the death or harmful bacterial infection caused by low levels of O2, improper maintenance and poor feeding routine. Finally, too much feeding, high water temperatures as well as chemical poisoning may have caused the death.