Fizz Lore Theory
One day, Fizz returned to find the city abandoned: his people had vanished, leaving Fizz without a clue to explain their disappearance.
I just can't think of anything else; Everyone in this deep ocean fortress, that's supposed to be inpenetrable, just vanishing like this?
Tahm Kench.
Tahm travel trought water, He could easily have made a deal with an important fish guy over there; Tahm could have easily just engulf everyone inside the city once the favor was done.
Your taughts?
ulted them all, for all we care.