In "The Bad Guy Club for Villains", he has a shark fin, two fish hands and legs, human hands, two alien eyes, and has a shark tail. In "Mermaid Man and Barnacle Boy II," he is gray, has a green shirt with checkered pants and a black hat on his head.
The Atomic Flounder's atomic ray, which was seen in "Mermaid Man and Barnacle Boy II," is possibly a reference to Godzilla.
He was actually a fish in a lab experiment but it went haywire and he became the villain he is now.
The Atomic Flounder is the only retired villain and the only one who has aged besides Viro Regnato.