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.