Wow bro, im so sorry for your loss. Possibly some bacterial infection that you dont notice unless you watch the fish for some time.
Ofcourse, there is always a slight possibility of damage done from outside the tank, some toxic material accidently got in, like something to clean the tanks glass.
Ofcourse, there is always a slight possibility of damage done from outside the tank, some toxic material accidently got in, like something to clean the tanks glass.