I got the Striped Marlin just from taking a picture of it. (Sounds like a catch-22, but I just randomly saw a few Marlin swimming around my coral reef, and I was curious, so I snapped some pics of them, which accidentally unlocked them.)

I don't know how or when the school of Marlin decide to visit, but be on the lookout for them. Snap a pic of them quickly and that unlocks them!

I got the Convict Tang by "doing nothing for 1 hour." (I left the app open and running for an hour without touching my phone at all)