First, you should go to the Temple of Time in the Black Forest and talk to the Statue of Liberty. She will guide you to do what you tell her. After getting the qualification certificate, you can follow the following rules.

Come to the sea ice spring square

Enter the kissing fish spring

Wait for the mermaid to appear (it must be a mermaid, not a shemale! )

The appearance of mermaid

Then you will get a fish and put it in your pasture and fish pond!