My goldfish is turning black!!!!Help?

your fish sounds like it's dying from the plastic... i've had many fish die from using plastic in the tanks, each one of them have died.... they turn black, and then you will see it's fins start to fall off. I recommend getting a bowl, or doing a 95% water change, that way you don't stress the fish out by moving it. Otherwise, i'd say a 100% water change.