Thats insane. Only time i have seen half black fish like that, some asian vendor years ago was doing some chemical dip of some sort to halves of fish to basically chemical burn half of the fish black. Not sure if anyone else on here remembers this. Anyway, do you think possibly he got caught behind your heater or something? Just throwing a guess out there.
I was told he may have slammed into the glass causing nerve damage so i have massaged him daily and slowly the black is fading, his appetite is great but he's still not back to his normal self yet, i will update his progress

