I agree with Snipe that there is a cause and effect. The cause my still be there or long gone.

Not qualified to have much of an opinion, but I wonder if there could be a bacterial infection involved. Do you feed your fish? If so you might consider lacing the feed with an antibiotic. We need one of the gurus to chime in.
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4 acre pond 32 ft deep within East Texas (Livingston) timber ranch. Filled (to the top of an almost finished dam) by Hurricane Harvey 9/17. Stocked with FHM, CNBG, RES 10/17. Added 35lbs RSC 3/18. 400 N LMB fingerlings 6/18