7 goldfish have died simultaneously in one day. Why?

You did not say how big the tank is and how often you change water. Fish excrete ammonia. Technically they are living in a toilet bowl. With goldfish you really have to change the 1/3 of the water every week. Goldfish are unsually tougher than tropical fish, but the fancy ones are delicate. Seven goldfish in a tank are too many. Do not mix goldfish with tropical fish. I have only one goldfish in a 30 gallon tank. It is almost twelve inch long.