whats this

[Wandering_Angler]

I think actually it is a comet goldfish mixed with koi. A fantail has the tail fin split into three where as a comet just has the usual two forks but elongated. Niether fantail or comet can get this big and comets are usually either all red/gold or red/gold mixed with white (in which case that are known as sarasa comets). Mixed with a koi this size and colour would be likely together with the fin array displayed by this fish. Hope this helps :)

[luke the carpy]

nice fish.
it looks like a golden carp (not goldfish) crossed with a koi.

IT COULD JUST BE A SMALL KOI, SOME JAPANESE ONES ARE VERY DARK

[NICK R]

It looks like some one has emptied their pond and placed it in the lake ,
A good way of transfering Koi Herpes Virus and wiping the rest of the stock out