As far re as I can tell these are all of the hidden fish I could find that have not been commented on above
Sorry still don't know how to get them
Here is the list (not in any specific order):
Cross Damsel
Jewel Damsel
French Angel
Springer dottyback
Spotted Mandarin dragonet
Convict Tang
Camel Cowfish
Juvenile spotted boxfish
Potbelly seahorse
Mauve stinger
Leatherback Turtle
Commerson Dolphin
Narwhal
Manta Ray
Hammerhead shark
And Whale Shark
Look under Manage fish to see what they look like
Spotted Mandarin Dragonet is my favorite

Just received the clown tang
Thanks
Also there is the
Pajama Cardinal
You get that by visiting the game 20 times in a day

Blue Clownfish
Randall's Goby
Those are all I have
Started the game yesterday sorry everybody