Fish are dying while wintering?

[crsublette]

I have tested the water and all levels are good:
PH - 6.5; Nitrates - 0, Ammonia - 0, Copper - .005; GH Hardiness - 180; KH Carbonate - 90
Temperature: 36 F at top 6".

Hmmm. KH and pH are dangerously low if this is an outdoor pond without any other supplemental materials to help maintain stability.
A very low pH also affects the koi's gills so this might by why they're staying near at the top since the O2 saturation is higher there.
I have never read anything of koi being healthy in waters below 7pH. They might survive at a 6.5pH, but I have never read such of a low pH recommended as "healthy". Google "koi acidosis".
With a 5dKH (90mg/L carbonate alkalinity) and a 6.5pH, then I would suspect one of two things: 1) test kit is inaccurate; 2) the pH is noticebly swinging. With the KH that low and, if you're water is only 36*F and you're pH is already noticeably swinging, then CO2 is likely the culprit for the swing. Fish, algae, and other microorganisms continue to respire CO2 even in very cold temperatuers. If nothing is changed, then pH swing will become much worse once the water warms up. With a 5dKH alkalinity, CO2 very noticeably makes a more profound diurnal pH swings. If the water warms up, the swing could become so bad causing the pH to possibly dip to 5.5 causing a pH crash.
I would definitely be looking at puting a good layer of crushed coral or Lithaqua or some aragonite material in your filters or in a stream or wherever there is a high water flow. With that low of a pH, this stuff will dissolve more quickly and will maintain the pH in the mid 7s (7.3~7.6) with an alkalinity at around 120mg/L. You can try baking soda, but I would not try it since it would cause too much of a quick change in KH while the fish is still in a more dormant state due to the cold temperatures.

Since most bacteria and parasites are dead or hibernating at that low of a water temperature (36*F), then I bet ya the pH is the cause of the fish kill.

 

[Dave 54]

We have a saying in the koi world and it is a good one, " become a keeper of water before you become a keeper of koi".
If in any doubt about water conditions try a half water change investigate where you are going wrong check your filters check your pond try to illiminate each from your list of suspects.
If still in doubt try another waterchange .
The saying is by Manky Sanke British water expert , the advice about if in doubt from the Koi International organization, both good advice.
I notice your testing for copper may I ask what test kit your using ( it's not a swimming pool one is it ) its just that all the test kits weve ever use never mention testing for copper.
rgrds

Dave