Some help please...cold water deaths!!!

[Brutuscz]

It is cold here in new england!!! The surface started to freeze and I have 2 large air pumps with 4 airstones bubbling just below the surface. Nothing new..have done it like this for years. Well..Today, I saw 2 koi laying on the bottom on their sides. I netted the first..and the gills were still moving. I brought him inside for my quarantine tank. Floated the bag..and dead within 2 hrs. Netted the second and already dead. Both these fish had survived the winter last season with no problems. I measured the water..ph=7.6, ammonia=0, nitrite=0. I add baking soda weekly to prevent a ph crash on my very soft water(gh40-60, kh-20-40). I have one other koi that doesn't look great....but all the others are fine. I've never had this happen at this time of year?? What the heck could it be??? I'm definitely at a loss.