...well, at least now I know what it is. I've never seen ANYTHING this virulent. Un-flipping-believable.
Late last night, I had a ~9month blue delta tail showing early signs of this. Lethargic but still taking food, what initially looked like columnaris on his caudal. This morning, most of his caudal and dorsal appears stiff and ashy grey/black. Started salt + Furan2 at 4PM. The necrotic(?) areas have completely disintegrated now, looking grim but trying to continue treatment.
Hindsight being 20/20, this probably is what killed my female a couple days ago. Watching my teal marbles like a hawk now... This is going to require the "nuke-'em-from-orbit" option for cleanup, isn't it? GRRRRRRR.