Help, have healthy ghost catfish but one is turning black???

I've owned them and they have never turned black on me

I've had quite a few of them (still have 5) and have also never seen or heard of them turning black. When they die, they turn from clear to an opaque white. That's weird. Maybe you want to post this at planetcatfish.com's forum, under Asian catfish? they're really the catfish experts.