You can start working on this upon picking up a Rumor pertaining to a mysterious fisherman. It’s Skoova, and you can find him in the Foothill Falls region on Koboh. Talk to him, and you’ll complete the Rumor, recruiting him for Pyloon’s Saloon, where he’ll look after your fish tank. You’ll also subsequently find Skoova in various locations, where talking to him will prompt him to dive and throw up a new fish to add to your aquarium. There are 12 fish to track down in total.

Full Skoova Fish Aquarium Guide