koi fish all of a sudden dying in outside pond

Question
We have an established 5 year old pond, 4 feet deep and easily 3,000 gallons of water.  The koi always stay outside in the winter. We are in WI and it freezes and thaws all winter.  We have lost 5 koi in the last 4 days, our largest Jerome died today, Jerome was 13".  Why after all this time would this be happening?  We checked the PH it's normal.  The water falls run all winter, with an extra fountain shooting water up and we have a heated ring to make sure a hole is always open on the surface of the ice.  The ice came and went 3 times in the past week.  We have an aqua scape system and about 30 fish still in the pond.  I just am so sad to lose these fish and do not want to lose anymore. Please let me know what you think.

Answer
Dear Brenda,

Sorry for the delay as I did not receive the first notification.
As I think you nee to check the water completely as soon as possible.
Ammonia is special.
Also check PH value in the morning, afternoon and evening.
High level of ammonia or large fluctuations in PH can kill fish.
Also take scrapes from the koi to watch under microscope after looking closely to the live fish.
There can be numerous possibilities what can be the issue here.
However best way is to get A koi vet as soon as possible.
If it is bacterial the changes of more dead fish are possible!
I do not want to take A risk here as more koi died.
The sooner professional help is in place, the better.
OR if you are an expert koikeeper, please take tests and scrapes as soon as possible.
Good luck!

Kind regards,
Raf