Diamond Watchman Goby has disappered!

[FitoPharmer]

Here are some questions, might help you narrow the disappearance down:
Do you have the prefilter on your AC or is it ran open?
How long has the yellow watchman been in the tank?
Was the goby eating any food while he was alive?
The tank is 25 gallons?
Where is your live rock from? was it "fresh" from store? ever hear snaps or pops?
Where is your bubble tip amenone in the tank?
When you setup your tank did you place the rock on the bottom of your tank or on top of the sand bed?
No hermit crabs?