My Yellow Tang Died Today HELP!

[Coldwater]

As the title says. I woke up today and BAM I see my yellow tang upside being devowered by my four hermit crabs. What happened!? It was perfectly fine yesturday. I am so confused.

Water Parameters

Ph- 8.2
Salinity- .24
Nitrite- o ppm
Nitrate- 0 ppm
Ammonia- oppm

What, my parameters are great!? How come he died? I had it for about eight months. Yesturady I added a Tiger Cowrie, Hairy Leg Hermit and two unknow hermits. There are no signs of it fighting with something on its skin. Any ideas what killed it?

Matt