Saw some defensive behavior between them and nearby sunfish. The pair here stuck together and lazily swam around the same area. In spite of their coloration, I dont think these are goldfish or carp, based off the body shape.

UPDATE: Marking as wild since I read you can mark escaped pets as wild on forums.

An escape most likely, but a nice unexpected one.
They're not known for their vagrancy potential, yet Alaska - Russia - Japanese East coast doesn't seem that odd for a soarer like this*. There are some previous records in the area and Kyushu from earlier this year. Making a possible autumn migration left-over an option (if it was not considered 'escape' or ship-assisted)
Not being familiar enough with the bird of prey options in Japan, I was stunned at first to see this bird in a skirmish with a kite over some prey-item.

*Apparently they're really only soaring migrants, so open 'cold' bodies of water may be difficult obstacles for them.

edit: February 2024: I only now see the records in the same area in April/May on ebird.