Once you have two adult sea animals settled in the same reservoir, you can raise a baby sea animal.

Tap on the Oceanarium, find a reservoir with two adult sea animals and tap the "Feed" button.

Wait until the feeding bar is filled and the growing timer starts counting down.

When the timer runs out, you will need to claim the new baby.

Don't forget that until the baby is claimed, it won't add to your Box Office profit.