I would think that if that goldfish was in there for any amount of time it would have gone back to a more natural brass/brown color. Maybe it outgrew a tank and was dumped recently. In fish tanks, when multiple different types of animals die (inverts, fish, reptiles, etc) it's usually environmental conditions like water quality and temperature. . If it was a disease affecting a fish, I'm not so sure it would affect the turtle and the goose. Maybe some one was fishing before you and they just happened to hate turtles, catfish and goldfish...and the goose was the only witness.