I'm no expert here but since you have had no replies...

I have a 3-4 year old rather large goldfish who likes to hang out at the top of my tank. He's spend a few summers in my pond but has resided inside for a year now. Last fall he started to hang on the top with his fin sticking out.

He shows no other signs of distress, eats well and swims down when I come into the room.

I have no explanation for it other then I think he likes it up there.

thanks ernie for the reply, at least one other fish does the same thing and is still swimming around.

I have no koi, but 7 gold fish of my little pond now downstairs in a 55 gal aquarium for winter. One of the large orange goldfish (female I believe) after eating pellets always hangs on the surface and gasps. I thought he might have simply eaten too much and the pellets expand in his stomach. But the other orange large goldfish (male) does not do it. So I thought perhaps there is an infection on this fish, and I applied half dose of Jungle Ick Clear, and he no longer is at the surface.