Dead fish ... cannot find the reason

[foxmulder]

This morning I've found one of my fish dead, my Salarias fasciatus, and I cannot understand why.
I have a 40g reef tank. The system is 8 months old, the water parameters are ok and the other tank mates seem to do fine.
I have another 2 occellaris clowns, 2 dartfish (Nemateleotris decora), 1 yellow tang, 1 cleaner shrimp, 1 bubble tip anemone (very small), and corals (goniopora, platygyra, fungia, scolymia, zoanthus).
I've had this fish for 4 months. It was showing no sign of disease. It was eating like crazy (evan last evening I've seen it eat).
The fish showed no sign of injury.... just it was smelling (probably because it started to decompose, although it seems early for this to happen).

What can be the reason? What could I have done wrong?

Thanks!

[ToddTT]

When did you test your water? Have you tested it today, after the death?

What have you done recently? Water change, new tankmates, decorations, etc?

In addition to the stickies here, this page has a quick rundown of common reasons:

Why Did My Fish Die?

Keep an extra close eye on your tank for the next week or two! And good luck.

[ToddTT]

Lots more:

sudden fish death reason saltwater - Bing

[foxmulder]

I tested today.
The results are:
NH3 - 0
NO2 - 0.02 ppm
NO3 - 5 ppm
KH - 6
PH - 8.2
CA - 400 mg/l
MG - 1200 mg/l

[leebca]

Sometimes it's just not something you did or didn't do. The tank inhabitants aren't the best for that sized aquarium; I don't know the foods you feed; I don't know how well you maintain the water quality. However, on top of the 'usual' things that can go wrong, sometimes the fish doesn't acclimate fully to captive life up to several months.

[foxmulder]

Ok. I hope I will not lose more fish.
Thanks for the answers!