An unexpected demise of a Goldfish after a water change can most likely be attributed to a drastic shift in water parameters. Sudden changes in water temperature, pH, or chlorine content can induce stress, leading to shock and possibly death.
Another factor could be the abrupt removal of beneficial bacteria if the gravel or filters were overly cleaned, disrupting the tank’s natural balance. To avoid such scenarios with your Goldfish, always ensure to match the new water’s temperature and treat tap water to remove harmful chemicals before introducing it to the tank.