I know in the aquarium world this condition is often referred to as Fish TB. While not actually TB, it is a closely related mycobacterium, I think. No idea if that's what this is, just know that this, or a similar condition, occurs fairly often in the fish trade.
Someone here told me (several years ago) might be vitamin C deficiency. I quit saving pellets over winter (vitamin C breaks down) and haven't seen it in quite awhile...of course I had to quit pellet feeding this year as my (new this past spring) grass carp were getting lazy and eating pellets instead of coontail. Just a thought..
In a lifetime, the average driver will honk 15,250 times. My wife figures I'm due to die any day now...
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