Our healthy 10 acre runoff fed pond shrunk to 1 acre through last years drought but we had good survival on BG, LMB, and CC as well as some 30 lb grass carp.
While the pond was low, we brought in a track hoe, rebuilt the dam and dug out some silted-in edges to about 5 ft to eliminate some cattails.
March and early April brought rain that partially filled about 5 acres. The fish were doing fine, BG & CC feeding at the feeder and catching good LMB. Late April brought more rain that filled the entire pond. Soon after, BG and LMB started dying. Now the grass carp and CC have started dying. I see no sign of any moving fish in the pond. No surfacing, no feeding at the feeder, no fathead minnow activity. My fish finder shows a few hits now and then, but not like normal. Something bad is happening in my pond.
Yesterday, there was a brown-green scum over about 10% of the pond with thousands of dead mosquito larvae in the scum. No chemicals nearby. The game warden came and took oxygen and ph level - checks OK.
This is extreme red dirt (NW Oklahoma) with lots of blue gypsum in the silt. Could there be a chemical reaction in the freshly dug out areas of the pond?
Any ideas?

Western Oklahoma. 15 acre 60 year old pond. Well established with fathead, bluegill, bass and a few large catfish. Aeration in the deepest (14 ft.) 1/3 of the pond. Most of the pond is 6 feet. Feeder.