On the night after Nemo was dumped into the fish tank, Gill, during an initiation ceremony atop Mt. Wannahockaloogie, tells Nemo (who was given the nickname "Sharkbait") that the Tank Gang was actually trying to escape the fish tank they have been contained in after Dr. Sherman's niece Darla accidentally killed a former member named Chuckles by shaking his bag on her birthday. He tells Nemo that the only way for the fish to escape the tank is to have Nemo jam the aquarium filter's motor with a pebble (as he is the only fish that is small enough to enter and exit the said filter), causing the entire tank to be completely covered with algae, and therefore forcing the dentist to clean the entire tank, at which he puts all the fish into plastic bags, which the fish use to roll out an open window overlooking a street next to the harbor and allowing them to escape.
The next day, the Tank Gang make their first attempt to jam the filter, but the pebble is unfortunately not jammed hard enough. And when the filter starts up again, the Tank Gang had to pull Nemo out of the filter with a decorative plant to prevent him from being shredded alive. Gill then feels sorry for almost putting Nemo's life at risk because he was too excited from going back into the ocean. But after hearing Nigel the pelican tell him that his father Marlin and his new friend Dory are coming to his rescue, Nemo decides to jam the filter again, but he finally succeeds this time. The fish tank immediately gets covered up with algae, with the filter completely damaged by the fish.
But unfortunately for them, the dentist replaces the old filter with a new one called the AquaScum 2003, which is equipped with a laser scanner that examines the tank every five minutes and a computerized voice telling the water's temperature and pH level, but no moving parts for the fish to interfere with, while they were sleeping on the day Darla was supposed to arrive. The Tank Gang react to the AquaScum with disgust. But after a brief "false alarm" as a result of the fish mistaking a young boy and his mother for Darla, the dentist scoops Nemo out of the tank with a net, causing them to react with shock and get Nemo out by weighing down the net. But the dentist still gets Nemo out anyway, but this time with a plastic bag. Nemo attempts to roll the bag out the window, but the dentist stops him by putting him in a box, just as Darla finally arrives for real.
During Darla's checkup, Nemo attempts to play dead to trick the dentist into flushing him down the toilet, but he attempts to throw Nemo in the trash instead when Nigel, carrying Marlin and Dory, flies into the dentist office and attacks the dentist, only to be shooed out (and Marlin thinking that Nemo is dead). Darla then picks up the bag containing Nemo, and begins to shake it. But upon realizing that such will kill him, Gill finally has enough and launches himself out of the tank using Mt. Wannahockaloogie and onto Darla's head, causing her to drop the bag onto a dental mirror and therefore popping it open, and Gill immediately knocks Nemo into a spit sink before finally being put back into the tank and celebrating with the rest of the Tank Gang as Darla gets her face sprayed by the spit sink.
At the very end of the film, the gang escapes from the office in plastic bags, make their way across the street, and jump into the ocean waters, although they remain floating in the water, and it is unknown what happens to them after that, although the ending credits suggest that they eventually managed to escape.
The Tank Gang only appear in the post-credits scene of Finding Dory, during which they arrive just outside the Marine Life Institute still inside their plastic bags now covered with algae. They are then scooped up by the institute volunteers.
The gang consists of:
- Gill (leader)
- Nemo (newcomer, only briefly)
- Chuckles (was likely a member, killed by Darla)
- They had a larger role in Finding Dory where they would help Nemo and Marlin find Dory and a deleted scene shows Jacques and Peach invade a ship to use the coordinates to locate Dory in the MLI in California. Andrew Stanton compared their role to Mission Impossible. Unfortunately their part was written out, but only since it didn't allow Dory to be the focus. It's highly likely that they'll have a larger role in the speculated third film.