First of all I want to say that the crisis is over and Sushi is fine. I'm just not sure exactly what happened.
I had him floating in a cup of his old water to get him used to the temperature of the new water in his tank. I let him float for a few minutes and put in a few spoonfuls of the new water. But I forgot to compare the temps, and I didn't see that the temperature in his new tank was about three degrees lower than the temperature in his cup of old water. I put him in his clean tank.
Now Sushi is a hardy fish. He's had quite a few brushes with death and come out happy and wanting to be fed. But he shot straight to the bottom of the tank, pressed his face between the power cord to his heater and the tank wall, and just sat on the bottom. He was clamping and he got really really pale, to where I could clearly see his stomach and air bladder. I couldn't see any gill movement at all, but his eyes would move every once in a while.
My first thought was to leave him alone for a few moments. When my fish have been in shock in the past, they do best with some space and dim lights. Then I realized that I hadn't got the temperatures the same. It seemed odd to me that such a tough guy would be so affected by a few degrees, since there have been times when I didn't get the temps right and he was fine after a few minutes of clamping.
He still wasn't breathing after about thirty seconds, although he would occasionally stir a fin. I wondered if he wasn't having some sort of hypothermic episode, even though the water was 76 degrees. I ran a washcloth under steaming hot water, squeezed it out, and pressed it against the side of the tank he was leaning against. After about five seconds he moved, started breathing again, colored up, and swam up for air. For some reason, when he took a breath one of the pellets I had fed him fifteen minutes earlier popped out. But he didn't have any issue eating it again.
Then he went into another sort of episode, sinking down to the bottom again (not tail downwards). This time he was in the middle of the tank, so I poured a few cupfuls of the steaming water into the tank. Boom, instant happy fish.
What on earth happened?