3/4 acre pond, 9’ deep, western N.Y.

Had a bad fish kill a couple years ago, was advised that my aerator was too deep (7’). So now I have it set at 4’ for the winter and run it about 25 minutes a day.
Had about 40 fish dead this week, also have some that are beaching themselves, gasping for oxygen, but just a few of those so far.
Did I not run the aerator long enough, too much?
The ice has only been off for a couple days, should I run the aerator more or?

Once ice is off there should be no problems finding oxygen as long as there is surface wind hitting your pond. You are not running it very long (25 minutes) and in 4' of water they should be able to find other warm water in the depths so that seems to rule out 'supercooling'

What type of fish are beaching or gasping? Small, large? all the same size class or mixed classes?

I'm wondering if you have a source of sulfide gas or methane gas being released making them suffer from the toxin effect rather than lack of oxygen? That might explain the last winter kill as well if it occurred around the same time of year?

Tyler, what species of fish are dying? Is it just one species, or several? Some are more sensitive to anoxia than others.

Is the diffuser in the same place or did you move it? How big of a hole was kept open in the pond compared to the total surface area (percentage wise)?

Yes, the die-off occurred at ice out or before last time. Mostly large bluegills with a few large shiners, only other fish in the pond are some perch.

The diffuser was in the same area as in previous years, only moved in toward shore to 4’ of water instead of 7’.

The hole in the ice was seldom larger than about 2 1/2 feet. Many times this winter, the hole froze back over within half an hour of the aerator being shut off.

I run diffusers through winter, 24-7, ice never freezes over completely. Cut back from three to two during this time, slightly deeper than yours. Bass, bluegill, perch all do fine, this winter was a cold one and zero die offs.

Tyler,

Do you have a lot of vegetation in the pond going into the winter freeze? This sound very similar to rapid DO depletion due to decomposition as the water warms.

That's my thoughts too. That situation (DO depletion due to decomp) could have been exacerbated due to ice and snow cover depleting the DO during the winter and the rapid decomp finally tipped the scale.

Lots of leaves, pond surrounded by trees, some weeds but not too many.

Should I start running my aerator more or ?

Thank you all for the help so far.

Here in Colorado I run my aerator, a simple piece of PVC with 1/16th" holes drilled in it, 24/7 in winter. Pond is 9' deep at beginning of winter and drains down to 5' deep by spring. Bubbler device is set in 5' of water at startup in November, which becomes 1' by spring. Weeds are very abundant in fall at ice up.
Trout, LMB, BG, GSF, YP, TGCarp, assorted other oddities all survive just fine.
It's very possible the shallow water gets super cooled by this winter mixing. My fish don't take any kind of bait or lures during the winter, but arrive at the dock the next spring for pellets and mealworms. In 12 years I've only seen 2 dead floating trout and no other fish. No fungus on those warm water fish.
If it were my pond, and of course it's not, I'd run that aerator constantly in the winter, keeping a reasonable hole open throughout the winter. The hole would be close to shore so that any creature falling in could access shallow water by the bank to climb back out. One of our house cats, while prowling (looking for trouble) out on the deteriorating ice in the middle of the pond one spring, fell through. He broke ice for a few seconds, then managed to clamber back on to the surface. Oh, the looks of disgust I got when I was laughing at him as he dripped up on to the dry ground.