The black-and-white damselfish in the tank is named Deb.
Deb is one of the “Tank Gang” fish living in dentist Philip Sherman’s aquarium in Finding Nemo (2003). She is identified as a striped damselfish and joins Gill, Peach, Gurgle, Bloat, Bubbles, and Jacques in helping Nemo plan an escape from the tank.
A running joke gives Deb an apparent companion: she calls her reflection “Flo” and believes it is her twin sister. Flo is therefore not a separate fish in the tank, which is why Deb is the canonical answer to the question. This detail can cause confusion when viewers remember both names.
Deb was voiced by Vicki Lewis in Finding Nemo. The Tank Gang’s escape plan eventually involves clogging the aquarium filter, though the dentist’s replacement filtration system delays their freedom. In the mid-credits scene, the group finally escapes into Sydney Harbour, still sealed in their plastic bags.