An Unpleasant First makes for a Crappy Monday

[SmallFry]

So today I lost a fish - the first since started this hobby about two years ago. Just as you'd expect it was one of my favourites too, Blueberry my Coral Beauty. We'd been away for a couple of days, but nothing unusual, got back on Saturday evening, everyone fine, still fine Sunday evening. Everyone eating just as usual. This evening I don't see Blueberry at feeding time, so I go looking and she's on her side behind a rock dead as a Dodo.

The only thing that changed is I brought some new food (Spectrum) for them back with me which they all seemed to like. I can't see that being the cause.

Everything is normal in the tank:
Ammonia - 0
Nitrate ~5-10ppm
pH - 7.8
Temp - 25.5C

The shrimps, royal gramma, wrasse (who somehow got into the 'sump' section of the tank - again) and the tomato clown (doesn't count because he's pure evil and I don't think the combined forces of NATO could kill him) are all fine and eating well.

My question I suppose is whether this a common thing, do fish just decide to die for no reason?

Can anyone suggest a possible cause - there seemed to be nothing at all wrong with any of them, and still doesn't for that matter?

How long does a Coral Beauty live - could it be old age (two years seems young to me though)?

I'm also a little suspicious that the clown may have had a hand in it since the wrasse was in the 'sump' again too, but they always seem to have held their own before, and she had no damage to her fins.

Finally what should I do now? With no sign of illness, no real warning and nothing obviously out of whack in there I'm at a bit of a loss where to go next.

Blueberry: