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

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..

Answer #1

so a few seconds of googling gave me these 2 possible answers:

either impurities in the water (ammonia for example) or over feeding

I would get a water test kit to see whats going on in that fish tank….

Answer #2

someone switched your fish

Answer #3

no it’s the same fish lol

Answer #4

so you think

Answer #5

so you think

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