The Fish Legions are massive armies of fish comprised of smaller fish swarms led by the Big Fish Boss. They travel electrical space currents to gain nourishment and propulsion, and can even harness it as a biological weapon. If enough space fish gather their electric charge, they can unleash a massive bolt of lightning that can eradicate any planet or source of life. The Big Fish Boss had "brainwashed" them to make sure the Galaxy remains a fish-only place. To do so, they needed to unleash a "Final Charge" that could possibly eradicate the rest of the Galaxy, leaving it only to the fish. However, Ratboy Genius' Starship absorbed the impact of the lightning and rediracted it into another dimension, foiling their plot.

One of the Fish Legions' natural predators is a species of space animals known as Fisheaters. However, other aliens, such as the Galactic Superheroes and Young Mouthbottom , also eat space fish when not properly settled on a habitable planet (for instance, the Galactic Superheroes live in their spaceship all the time).