aguita,
Short summary:

The first time it happened, I took the blame for not moving the air stones in close to shore and figured that the water was too cold from circulation.
None of the other fish suffered and the HSB in our shallower ponds (7' to 9') deep survived just fine.
Do all ponds have aeration? Only the pond that had the die off has aeration.
How old, how big and how many HSB were in the 3 acre pond before the kill as compared to your other ponds? 50 fish, three years old and most were near 20" as far as i could tell.
Are the HSB in the 2 other ponds the same age and size as the ones in the 3 acre pond? They are the same age
What is the size of the other 2 ponds? One is 1/4 acre and the other is 3

You have 3 ponds (1/4acre and 2 three acre ponds) and HSB are only dying in the pond that has aeration. I’m no expert but it sounds like the problem is the aeration. This leaves you 3 options if you want to keep HSB in that pond.

1. Turn off the aeration and see what happens. If it’s like your other 2 ponds the HSB should be just fine.
2. Have an aeration expert from a pond management company come out and look for the solution.
3. Reread this article
https://www.pondboss.com/news/moderators-corner/to-aerate-or-not___is-it-really-a-question
and ask esshup (http://www.hoosierpondpros.com/) questions. He wrote the article on aeration.
I hope this helps you aguita!