It's been a very long time since I've spent time in this forum and generally that is a good things and it meant my pond was developing and maturing and taking care of itself. Unfortunately my pond has fallen on hard times. Here's the back story and hopefully someone can offer some advice. My pond is 10 years old. It has about 3/4 acre of surface water and is kidney shaped with two 10-11' deep pools and a 6' deep center section. It hold water very well. A full bore garden hose is too much for it to handle. It was flourishing with bluegill and bass as well as a handful of large grass carp this spring. Water clarity was excellent with little or no algae to speak of. On the night of Tuesday, June 14 we had a nasty storm come through that dropped 3" of rain in about 75 minutes along with a large amount of large hail. Generally speaking my pond doesn't see much runoff water but big events do cause some issues. Water ran through the pond for about 10 hours left it looking like a milk shake. I was worried I'd have to treat with alum again to settle the clay particles. I had done that about 6 years ago with great success. Thursday morning we woke up to hundreds of dead bluegill around the pond. For the next three days we cleaned up dead fish. Many hundreds, maybe thousands, of fish died. First it was the grass carp and large bluegill, then the large bass, then smaller bluegill and so on. Not many crappie (stocked 100 about 2 years ago) were found dead. We also didn't find any small bluegill or bass that should have spawned this spring. I have ran my aerators non-since and borrowed a circulator for two weeks. Water clarity quickly improved and looked good a week later. To date, I have not seen a single fish, dead or alive, since the storm. Nothing comes to feed or bites when fishing. To make matters worse, duckweed and algae are covering 90% of my surface area. Aerators still run dusk to dawn and I installed a 3/4hp circulator two days ago. Under the duckweed the water is crystal clear....actually better than I've ever seen it.

I can't believe all my fish are dead. Any thoughts?

I'm guessing some of your smaller fish survived. I would do a seine survey and look for survivors.

Sounds like you had an anoxic layer form in the deepest parts and when the cold rain came and caused runoff it flipped, mixed and killed a lot of fish. Combination of the hydrogen sulfide, low DO and Temp shock is not good.
As Shorty suggested, I'd bet there are some smaller, healthy individuals that made it through.

If that is the cause, would you expect to see fish gasping at the surface? (Higher DO, warmer water.)

If a pond owner did observe that (or just had an event similar to what Golfnut described), would there be any preventative measures that could be taken to avoid or lessen a fish kill?

For 3-4 days I did have fish at the surface gasping for oxygen. I would agree that my pond flipped over due to the influx of cold water. The runoff probably had higher concentrations of nutrients that one would like. I'm in farm country.

I'd guess that all of the people active on this thread are within 200 miles of each other. I'm west of York, Nebraska about 10 miles.

I think the washed in fertilizer would explain your bumper crop of duckweed and algae. However, I don't think that was a factor in the fish kill?

Your gasping fish seems to confirm the low dissolved oxygen problem.

Hopefully, some of the experts can drop in and give some advice on proper procedures after a similar future event to help you and others avoid a fish kill.

Sorry about the loss of your big fish. Hopefully you can post an update in the future reporting that you still have decent numbers of your small fish remaining!

The only way to minimize that from happening again is to run the aeration system long enough per day to homogenize the water column as much as possible from top to bottom.

+ 1 on what esshup stated and turning the water over aggressively can inhibit DW/Watermeal.

Piping at the surface was your advanced warning of what was about to happen. Rapid, intense agitation is in order if you ever see that again.
You are 4 hrs NE of me, Holler if you ever need a hand.