Ok some of you guys add a touch of salt, perhaps a tenth or two of a percent. This would lower the freezing point by how much? I dont know, but wont the water in the kois body freeze, even if the pond remains a liquid, isnt the freezing point on a celluar level just below 0, I think it was -2 centigrade. A totally fresh pond would freeze with the fish trapped in the ice, but a slightly salty pond would have frozen fish floating all over the surface since ice is lighter than water. I wonder how many of you have thought to get a big heater for the pond instead?

I've never used salt in my pond so I'm not sure what it would do. Fish basically go "dormant" once the water gets below 50. When my pond gets that cold the fish just hang out in the deepest part not moving. The key to keeping them alive it to make sure you keep a hole open to allow the gases to escape. Two years ago my fish survived fine but last year even with a heater I couldn't keep a hole open (200 inches of snow didn't help either).

Koi blood has a natural salinity level of .9%.
My thought is if you have too worry about the temps getting that low all of the fish are going too die anyway.

dont ponds and lakes in nature freeze solid? how then do not all fish die

If the water is deep enough then it doesn't freeze all the way through. My ice gets about 6 inches thick and along as I keep a hole open to let gases escape they are fine.

Like Debi said most ponds in nature do not freeze solid. And if they do freeze solid, YES, ALL OF THE FISH WILL BE DEAD!