Release fish from all the hidden pools.
The hidden pools are located in chapters 1, 2, 3, 4, and 6.
There are a total of 20 hidden pools located in this game. Some of the pools are easy to find and some you will have to go diving deep in the ocean to find. When you get near one of the hidden pools interact with it by pressing the button in order to release the fish from the pool.
Refer to the video below for more help on finding all of the hidden pools: