We had a few cold days here in the Toronto. Canada area a week or so ago
and despite my best efforts I could not keep a hole open on my covered pond.
last week when it warmed up and went out and cut a hole in the ice and
shined a flashlight into the deep area and saw one of my large 14" koi dead
on the bottom.

I had an airline running, but any moisture in the airline iced up and
blocked the air which is when my hole froze over.

Hi Joel - I am 35 miles east of Windsor and was having the same weather as
you. My airstone hole was freezing over like yours on the coldest days and
nights and I had to constantly keep opening the hole when that happened with
either a tea kettle or a steel rod - if you let the "ice dome" freeze over
solid it will start to thicken and be next to impossible to open - that
happened to me 2 yrs ago and I had to use a 1500 watt heater that I sat on
the ice and let it melt it's way through to open the pond back up
I also have my airpump 3' off the ground (on my waterfall "hill") and the
airline runs on a constant slope downwards with no low hanging loops for
moisture to collect in during thaw/freeze temp cycles, but we haven't had
any above freezing temps around here - have you? If not, I would suspect the
hole froze over solid enough to stop the airflow, rather than the line
freezing up
If you aren't able to check the air hole daily in the really cold temps,
maybe you will need to use a 1200 or 1500 watt heater on a timer for a
couple hours a day with your airstone
I have been using an air pump for 10 yrs now with the airstone no deeper
than 3" and it has worked fine for me even in the coldest winters, except
when I got lazy about keeping the hole open a couple yrs ago, although T.O.
has a bit cooler temps, I believe
It sounds like lack of oxygen, or rise in toxic gases as the largest fish
are the first to suffer in those cases
Sorry to hear about your losses
Gale :~)