Seven animals live in the dentist’s fish tank at the start of Finding Nemo.
Before Nemo arrives, the aquarium’s residents are Gill, Bloat, Bubbles, Deb, Gurgle, Peach, and Jacques. Together they are known as the Tank Gang and live in Philip Sherman’s dentist-office aquarium. Nemo becomes an eighth resident only after divers capture him and Sherman places him in the tank.
The count includes Jacques, who is a cleaner shrimp, and Peach, who is a starfish; it is not limited strictly to fish. The seven-member group also includes Gill the Moorish idol, Bloat the pufferfish, Bubbles the yellow tang, Deb the striped damselfish, and Gurgle the royal gramma.
A frequent counting mistake is to include Nemo, since he spends much of the aquarium storyline with the group. However, the question asks about the tank at the start, before Nemo is introduced to its residents. Counting the original Tank Gang gives seven. The group’s escape plot drives much of Nemo’s aquarium storyline, as they try to avoid being given away to Darla.