If required to take a photo or share something tap the icon get off, then tap it again and repeat until your requirements are completed.

Fish	How to unlock
Blue Clownfish	Take a photo of a clownfish 3 times.
Clown tang	Take a photo of a tang 5 times.
Convict tang	Do nothing for 1 hour.
Cross damsel	Take a photo of a damsel 5 times.
Giant Isopod	Play the game for 14 days.
Jewel Damsel	Tap the top left corner 1000 times.
Juvenile Spotted Boxfish	Start Application upon Notification.
Leatherback Turtle	Open mystery chest 50 times.
Narwhale	Tap the top left corner 5000 times.
Nautilus	Use Delicious Food skill 50 times.
Pajama Cardinal	Open the application 20 times in one day.
Randals Goby	Take a picture of a Goby 3 times.
Sea Slug	Play the game for 7 days.
Snowflake Clownfish	Tap the Twitter button 5 times in settings.
Spotted Mandarin Dragonet	Start Application upon Notification.
Springer Dottyback	Share a photo 3 times.