This issue seems to be related to MCPE-49607 and MCPE-39073.

Aquatic mobs seem to get stuck on a shelf when there are groups of them together.

When attempting to navigate a shelf, it appears Aquatic mobs attempt to path find to a specific point, but continuously collide with one another and the corner of the block making their direct line to the destination impossible to navigate.

To recreate, build an upside down L shape under water and put several aquatic mobs in the water. (I have only tested with tropical fish, but have observed this behavior with turtles and salmon as well)