First dead fish since October of last year :(

[George]

Sometime fish has internal damage or parasites that are not visible to us. Not sure if your banggai was wild caught or tank bred. If it's wild caught, then there is only one source of them which is Indonesia and that place is notoriously for using cyanide to catch marine fish. Cyanide damage fish can take months to die. Again not saying that is the problem but one of the possibilities.