They are a species of carp, other species of which are widely eaten.
Was there not a fad in punk times for swallowing the poor creatures live and whole?
A good big (dead) one would be nice baked in foil, lots of herbs, lemon etc.
Small ones could be battered and deep fried -- plenty of paprika and ground black pepper in the batter?
Koi carp are apparently eaten in Japan, as well as being highly admired (they are the same species as common carp). Apparently for a really good occasion such as a wedding, you are supposed to have one with a good pattern (and thus very expensive).
Poissonally I'd want my goldfish supper to have been fed on proper food, not the minced fish guts and tartrazine or whatever it is they put in fish-food.
I've only had proper carp once, and it was a bit "tired" so was not a rewarding experience. However, I've had chub (another carp-family freshwater fish) -- this is supposed to be very dull, but I found it excellent.