So I take a look at my tank this morning to see one of my green chromis dead and pinned to the side of a K4. No warning whatsoever. All params are within the normal levels, and I've not made any recent changes to the tank. The chromis is one of the original fish in the tank and has been there for almost 3 yrs. It has been eating well and has shown no unusual behavior. Body mass looked good for a slender fish like a chromis. No gill inflammation, no lesions, no bite marks, no torn tissue or missing scales (although spending the night stuck to the k4 roughed up one side, but no major damage was visible). The only thing I saw that was out of the ordinary was a small chunk missing from the gill cover on one side, but the gill underneath looked undamaged.
This was my first fish loss in over a year. It's bad enough when someone goes carpet-surfing, or if you find out about a hitchhiker crab when fish start disappearing. At least you know the cause then. I have NO idea why this little guy went belly up, and that always fires up the paranoia machine.