this sounds HORRIBLE but after he died i did an autpopsy(i used to work at a vet, its just something i do, i can't stand not exploring every option to know what i did wrong)
he had managed to eat a peice of carbon -__________________- wth..
so yea, he was blocked, with no relief, i honestly don't think i could have done anything about it so be SUPER CAREFUL you dont drop a piece of carbon in your tank if you have bigger mouth fish