Feed the Fishes is a location-based challenge in Borderlands: The Pre-Sequel. To perform this challenge in Hyperion Hub of Heroism, it is required to use the aquarium in the Crew Quarters. It is worth 5 Badass Rank.
The Crew Quarters is located on the northernmost part of Hyperion Hub of Heroism, where the entrance to Research and Development is located. There is a small fish tank on top of a desk, located up some small steps, to the right when facing the Dahl graffiti. When approaching the tank, the action "Feed fishes" becomes available. Simply perform the action to complete the challenge.