Experts will be here soon to help, I'm sure. In the meantime, sure sounds like you may be right. Hot water contains less DO, and all these factors put together might be the problem. If you've had some cloudy, calm days lately this isn't helpful, either. Well water is usually low in oxygen, by the way. Could be better to turn it off, or at least add oxygen to it.

If you have some kind of marine motor maybe you could run it to help oxygenate & aerate. And in your shoes I'd cut back on feeding, at least for now.