why did my emperor die?

[scooby8]

my favorite fish just died last night. he was a juv emperor that i had for about two months. his color was great and he had been doing very well.the day before he died i tested the water and here are the number.
ammonia---0
nitites --- 0
ph

[hr]
8.2
nitrates

[hr]
35-40
salinity

[hr]
1.0023
these numbers have been consistent for months in my tank. the other fish ,humma trigger and surgoen tank and dansell are fine.
there were no sighns of spots on the empereor.i came home for lunch and he was at the bottom breathing really hard .then when i came back 3 hours later he was dead. could it have been the romaine lettuce i feed him? he's ate it before and there never was a problem.........any ideas or is the life span for these fish not very long...
60 gal acrylic
wet dry system w/ skimmer
crushed coral bottom
thanks please give me some advice.......

 

[barracuda]

First of all, i'm very sorry for your loss
I had juv Emperor once and he also died. I think it maybe collection impact. If the fish was cought using cyanide it may die much later, after the shipping and aclimating. Mine also died w/o any known reason. But i would definitely bring those nitrates down though.