So sorry to hear he is gone
Any other info you can give so everyone can tryin help you find out what might have happened? When was your last WC? Not sure of your location but here in Michigan when the weather turns real cold they treat the water a bit harder at the plant so I always have to cut back on the size of my WC's in winter so I don't get such parameter changes (Ph and such). I just lost a 5" BGK last week 1 hour after doing a 75% WC because of that very reason.
Is there anything small in the tank he maybe could have ingested? How old/big was he and how long did you have him? Might have just had some issue you didn't even know about when you got him perhaps.