There were too many to list, but know that we appreciated the time you took to reply. In case any of you are curious, I'm including a brief synopsis of a couple replies that fit our condition. Also we have a continuous flow system, with incoming water going through several types of filters.
Cold water carries much more dissolved gas than warm water.
When the water is heated in a closed sysstem, the dissolved gases don't come out until they hit the aquarium, or the fish. The fish get bubbles, their eyes pop out and the fish can die. Avoid this problem by slowing down the water flow and by bubbling air in the tank at all times. It is something that hits different tanks differently because each tank equilibriates differently. We won't know if this will help until the next cold snap, which looks to be this weekend.