3°F here this morning and RES survive in my pond and the local lakes. I've experienced temps in the -20's and almost 3' of ice on the local lakes in previous years and the RES have survived. Now, that may be due to some underwater natural groundwater springs in the lake bottom, giving them a slightly warmer water refuge, but knowing what I do about groundwater DO, I don't know if the O2 levels would be high enough in the warmer water for them.
