Single Torch Head Died

[beanplanted]

Hello,

Tank: 20 gallon AIO
Parameters:
Ammonia: 0ppm
Nitrate: 20ppm
DKH: 9
Salinity: 1.025
Temp: 78*

My malaysian skittles torch ive had for about a month has been thriving. 2 days ago it looks healthy and colorful, and then I woke up this morning and an entire head has died off.

I have a 2 other torches in the tank and they are healthy, colorful, and thriving so I would be inclined to believe that if it was a water parameters problem they would also be struggling.

I have 2 emerald crabs in the tank, any thoughts on if they are the culprits?

I’m very confused how this happens, the other fish and corals look perfect and healthy, how does a head just die?

 

[RedReefer]

Post pic of tank so we can see agevand health…

 

[beanplanted]

Post pic of tank so we can see agevand health…

 

[Shirak]

Pictures with white lights please. Any up close shots of your torches showing the flesh band?
Could just be unlucky on that one and it got something fast acting like bjd

 

[jackintheboxfish]

What’s the phosphate level? My torch receded a lot when I was gone on vacation and phosphate bottomed out for a week.