To get one, you have to go in the scenery place and go to where the statues are. when you are far enough in the game, there will be a mermaid statue. Make a tank, it has to be as large as the number of mermaids you want. It has to be empty. Then select the mermaid statue and place it in the tank. It will start to shake, then it will break. You now have a mermaid tank. Assign 2 marine biologists to the tank and then you're done.

P.S: mermaids don't have babies!!! Good Luck!