Purple Firefish Goby - Swollen abdomen

[CraigMixedReef]

My Purple Firefish Goby became sick and died very quickly (within a few days). I rarely saw him (or her), as he liked to hide in the rocks. I found him on the bottom tonight, and upon inspection, noticed his side was very swollen and discolored. See the picture below, please. Healthy tank (so far) and all parameters are within normal ranges. Mixed reef environment.
I don't believe he was bullied. All other fish appear healthy and are eating and active. Any thought on what killed my friend, Mr. Goby?

 

[4FordFamily]

Looks more like rigor mortis than any ailment — IE how land animals decompose and get “fatter” due to gas buildup upon dying until they “burst” then they shrivel up and dry out. A gross topic, sorry.

I don’t know the science behind it but that’s what this looks like to me.

 

[CraigMixedReef]

Thanks for the quick reply.
He wasn't dead when I found him. He was still breathing at the bottom and upside down.
He died shortly after I noticed him.

 

[Humblefish]

His abdomen looks a bit swollen, but I'm not sure if that discoloration is a bruise or as mentioned above, rigor mortis. You'd have to perform a necropsy and examine his innards to know.