The fish who leads the tank gang is Gill.
Gill is a scarred Moorish idol who leads the group of aquarium fish living in dentist Philip Sherman’s office. The tank gang includes Peach, Deb, Gurgle, Bloat, Bubbles, and Jacques, and Nemo joins them after being captured and placed in the tank.
Gill is determined to escape captivity and devises a plan to help the fish reach Sydney Harbour. He encourages Nemo to clog the tank’s filter, although the attempt nearly injures Nemo and makes Gill regret involving him. Later, Gill helps Nemo escape through the dentist’s plumbing.
The answer is sometimes confused with one of the gang’s other members. Jacques is the cleaner shrimp, Peach is the starfish, and Bloat is the pufferfish. Gill is both a member and the recognized leader. Willem Dafoe provides his voice in the film.