I don't even know if it could have been avoided - I am totally speechless.

My zebbies were all fine, happily plodding along and eating tons of food. Last time I checked was yesterday morning. Last night I fed but as I was tired and flu-ish I didn't stay to observe, as I usually do. This morning I missed too - was late up and had to rush to work. On my return I went to feed and saw one of my beautiful males dead! He was in between some rocks and his cave, facing inwards (don't know if he was fending someone off or he got out of his cave to go in the rockery to hide. He has a bite mark you can see very clearly in one of the pictures, its a deep round hole at the top of his body near the bottom part of his pectoral fin - then the redening spreads around it. His head is a bit bruised over one eye. From a ventral view I can only see a bit of reddening/bruising under his pectoral fin. That's it. Otherwise untouched.
It seems to me there was a fight, but I can't think of anybody challenging him!! My other male zebra "lives" far away, at the centre of the tank! The other plecs don't fight with the zebras, never!
I am gutted .. he's been in the tank since he was in "nappies" and was now 8 cm SL.

I am posting the pictures in case somebody has an idea of what could have happened and when it could have happened .. did he die as a result of a wound inflicted earlier (say a couple of days ago) which got infected or was it a straight fight last night / this morning? The wound looks like a hole with bruising/infection?? around it - but its quite deep. Could he have hurt himself? Any ideas will help as I need to check what is happening, if all of a sudden there are fights in the tank!
Thanks