Feed the Fish is a location-based challenge in Borderlands: The Pre-Sequel. To perform this challenge in Research and Development, it is required to get an enemy to jump in the water. It is worth 5 Badass Rank.
This challenge is completable in the area connecting the Aquatic Observation Area to the Aquatic Life Stabilization Area in Research and Development, specifically around the U-shaped bend leading up to the Dahl troops.
There are many Torks in this area, and they will often end up jumping down into the water of their own accord without any player help. A few runthroughs of the level during the course of the main game and side missions will likely be sufficient to complete the challenge purely by accident.
A more direct way to complete the challenge is by using an Oz Kit's Gravity Slam to blow the Torks over the edge. Care must be taken to ensure that the Gravity Slam doesn't kill the Torks in question, though, so an underlevelled Oz Kit is more suitable for that purpose.