Firefish are a rather delicate and nervous fish species. They can often react quite adversely to new additions even if there is no aggression. Dartfish as a whole can react quite commonly by hiding, loss of appetite and death. Could simpley be the addition of the goby has "upset" their normal routine. Being your tank is also quite new, the quick addition of so many fish could have easily upset your biofilter. Can you post the exact numbers
for your water parameters?
Do you know the species name of the goby or post a pic? Many types of "bar" Amblygobius
are refered to as bullets. It may not make much diference, but you never know.
If anything, I would QT
the goby as apposed to the firefish. It will help determine if that is indeed the cause, although you would need to be very sure not to disrupt the tank further. Increasing a stressful situation could send the firefish beyond the point of recovery. FWIW
, it would have been the better route from to get go.