Here is an African folktale about ‘Why Hippos Don’t Eat Fish’:

When God was giving each animal a place in the world, the pair of hippos begged to be allowed to live in the cool water which they so dearly loved.

God looked at them, and was doubtful about letting them live in the water: their mouths were so large, their teeth so long and sharp, and their size and their appetites were so big, He was afraid that they would eat up all the fish. Besides, He had already granted the place to another predator – the crocodile. He couldn’t have two kinds of large, hungry animals living in the rivers. So God refused the hippos’ request, and told them that they could live out on the open plains.

At this news, the two hippos began to weep and wail, making the most awful noise. They pleaded and pleaded with God, who finally gave in. But He made the hippos promise that if they lived in the rivers, they must never harm a single fish. They were to eat grass instead. The Hippos promised solemnly, and rushed to the river, grunting with delight.

And to this day, hippos always scatter their dung on the river bank, so God can see that it contains no fish bones. And you can still hear them laughing with joy that they were allowed to live in the rivers after all.