Since I don't have a great amount of details to share, I am not expecting precise answers, but I am hoping that I can get some sense of what may have happened to a friend's pond.

The pond is thought to be about 14' deep, but only 1/3 acre in size. The previous owner was a fisherman and would bring back fish caught from other locations and stock them in this pond. There were mature catfish, crappie, LMB and bluegills present at a fair level.

Then about 3 weeks ago the pond "bloomed" and was 100% covered in what I would call "duck weed". About 2 weeks after the bloom, there were dead fish everywhere and plenty of them. My sense is that the complete coverage of the duck weed drew his DO levels down and the fish perished as a result. Does this seem plausible to experienced pond masters?

Yes.

Yep

Thanks guys...I found out a little more info. My friend had also treated the pond with copper sulfate powder about 2-3 days before the duck weed bloomed like crazy. He used about 2-1/2 pounds.

Could the treatment have spurred the radical duck weed bloom?

Thanks guys...I found out a little more info. My friend had also treated the pond with copper sulfate powder about 2-3 days before the duck weed bloomed like crazy. He used about 2-1/2 pounds.

Could the treatment have spurred the radical duck weed bloom?