Lost one of my cories today

He was the smallest and always a little less boisterous than the others. Yesterday afternoon I missed him at feeding time, and searched in vain all evening. This morning I found him still alive, but sticking to the filter intake. I put him in a QT by himself so he wouldn’t get picked on (not that there was a mark on him), and an hour later he was dead. I have no clue what killed him. All I can think of is that he was fighting something since I got him, and the stress of being moved to the main tank on Wednesday weakened him further. But I have no idea what he could have been fighting. Apart from the fact that he was half the size of the others (and they’re all not fully grown, so I thought he was just a bit younger), he didn’t look any different. He was definitely out and about and feeding with everyone else yesterday morning. Water tested the same as always (amm/trites 0, nitrates under 20).
Needless to say I’m keeping a sharp eye on the others now.