Why did my koi fish die suddenly?

There are several possible reasons why your koi fish may have died suddenly, including poor water quality, disease, stress, or inadequate nutrition. It is important to regularly monitor the water quality in your fish tank or pond, provide a balanced diet, and ensure that the fish are not overcrowded or exposed to sudden changes in temperature. Consulting with a veterinarian or fish expert may help determine the exact cause of your koi fish's sudden death.