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Why did my goldfish turn from bright orange to black & get 2x larger?

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About exactly a month ago, my family & I went on vacation for a week & left our neighbor to take care of most of our pets, except the fish because they had one of those food block things just sitting in there & yeah so my neighbor payed no attention to the fish. But anywhoo.,when we got back the fishes fins were like HUGE, the fish didn't much bigger just it's fins, & it was almost completely black. & now, a month later, its back to bright orange, but still huge. We thought it died & our neighbor replaced it, but no she didn't because if she did then it would of died because theres a beta fish in that tank & it kills other fish unless its the ones that were already in there, so no. Also the tank is next to a glass door & while we were on vacation the curtains were closed, but I thought if a goldfish was left in the dark it turns white, not black..