Goldfishes died on first night?

I bought two small goldfishes and had them in a classic round looking bowl, enough space for the two of them, and at 1 when I went to sleep they were perfectly fine, the water was extremely clear and when I woke up at 9 they were dead and the tank was greenish and you could hardly see through it. I know it might be that the water was bad quality, but I used the same water for my last fish and it was fine. So why did they die?