I'm pretty sure this is because the fish are trying to swim past the block, but their hitbox is longer down than their AI thinks it is, so they just get stuck. I'm trying to do the same thing with one of the walkthrough tunnel aquariums and i just ended up blocking off the sections so they wouldn't try to swim to the other side.
Luke Adams
MCPE-70260
I'm pretty sure this is because the fish are trying to swim past the block, but their hitbox is longer down than their AI thinks it is, so they just get stuck. I'm trying to do the same thing with one of the walkthrough tunnel aquariums and i just ended up blocking off the sections so they wouldn't try to swim to the other side.