Lonely Fisherman

20 different fish, and there are 25 in the game, so you've room to skip the fiddly ones. Fishing unlocks via the "Looking Glass" side quest at the Oasis spot in the Great Desert - talk to Clyde for the rod. The Atelier spot on Eidos 9 is missable (needs Lily's bar full, same gate as 968392933), but you don't need it to reach 20.