One of our very large Koi (approximately 8 years old) has been isolating itself in the same spot for about 10 days (nose/head in to the side of the pond). We cannot see any visible signs of infection or distress. It just hangs out there. Today, another of our large koi swam over to this koi, went underneath it and butted it with its nose and actually got this koi to leave its spot (I hope this makes sense). The koi that I'm worried about swam around the pond and looked just fine for a few minutes. The koi that butted it went into the spot that the first koi stays in. A few minutes later they switched and the original koi went nose in to that exact spot that it's been hanging out in. Does anyone have any clue as to what might be going on??? I hope I made some kind of sense to someone.

Thanks in advance.