Goldfish are essentially carp.
The Chinese initially reproduced them to eat.
Carp, which are ordinarily dim or green, breed like insane, and you get varieties of tones and shapes.
Nature messes about.
They have a sprinkling of shade cells that are red or gold.
A change would have smothered the dark color cells, permitting the yellow and red ones to be communicated.
People took a change and made a types of them.