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Bought fantail goldfish last night. One of them was really clear and pretty colors. Checked on them this morning and the big one is covered in black??? Will try to get pics! when it was bought now.
I have a very similar fantail goldfish, Spotty. Spotty often changes the intensity of her spots when she goes to sleep, so I don't think there is anything untoward there - although I would watch your little guy's water quality.
Here's Spotty with her pal, Goldie, eating cucumbers.