Created a new world on 1.13 and after a few days ALL aquatic passive mobs have vanished entirely.
I note a number of other people have reported the same issue yet they have all been marked as resolved, when the bug is still there.
I understand mobs will despawn, under the right criteria, but they should also spawn which they do not. Not even the squid. And this is the same anywhere in the world I visit.
Two things that may or may not have contributed to this issue.
i) I built an underwater kelp farm before the fish disappeared
ii) I captured some tropical fish and put them in an aquarium.
Even the Hermitcraft server is experiencing the same issue
I hope you will actually resolve this bug as it is just one of a plethora of bugs in 1.13.
Whilst I realise bugs are inevitable, the shear quantity of them in 1.13 is beyond belief. I hope this is not the sign of things to come.....