Back to the Past/The Bad Guy Club for Villains - S7-E16
Continuity mistake: The Quickster was a member of the I.J.L.S.A in Mermaid Man and Barnacle Boy V. But in this episode, there isn't The Quickster in the I.J.L.S.A, but instead the Pi-Right Ponderer.
Answer: I believe this is the Season 6 episode "Spongicus".