I’m not an expert, but from what I do know about jewels, is that they are extremely aggressive, especially when breeding is in their plans. Almost all the literature I have read about them says that if you have a pair breeding, that they will often kill other jewels in the tank. You said they were in a big tank, how big? I’m just wondering if you may have gotten away from territorial/spawning squabbles because of the size of the tank. Then one day this guy may have wandered a bit too far into another pair’s territory and got a bite on the gill?
I agree with
tlindsey
on keeping water pristine, if you can dose with some salt and keep an eye on it. Just my thoughts.