I have one too about 4" long and upper body gradually turn whitish and next thing i know it RIP. u might want to check the ph level, should be ard 6.9 to 7.4. though is hardy fish but somehow it didnt do well in my tank
sorry to hear ur lost man..it sucks when u have try everything to save him but still not enought. my ray died a week ago so i know how u feel. i wish theres special doctors for fishes lol. feel better and rip for ur sturgeon.