Our littlest, Rukia, that never grew much, that we tried specially hard to feed and help into her first winter. It was my fault, my inexperience. I saw her up swimming that one cold morning, and looking up the problem, found that it was a symptom of hypothermia: inability to keep her core temperature. She was only about 4″ long, and not our fattest, either. Makes me sad. I wondered then if I should pull her indoors, but I hesitated to go look up the problem, and then by the time I knew what to do, she had gone below to the rest of them, and died there. So sad. I’ll know in future and set up a rescue: I’d have had to get her a bucket of pond water and let it warm slowly to house temperature. But I didn’t. Bummer. We have one other fairly small one, and this confounded erratic weather just hasn’t helped the pond. But if I see this behavior in any other, that fish is coming inside.