The Fish Duel was an event that happened in "Dueling Eds." Rolf was the challenger of the duel, who wanted to regain his honor against Eddy after having been insulted with the desecration of a fish ball.
Celebrating how his Great Nano captured a dangerous sea cucumber many years before, Rolf makes some sea cucumber balls and serves them to his friends while wearing his sea cucumber suit. However, Eddy refuses to eat the sea cucumber ball given to him by Rolf and throws it at a nearby fence. Extremely offended, Rolf miserably walks away. Eddy is satisfied with what he just did to that disgusting sea cucumber ball and does not care about how much he emotionally hurt Rolfs feelings. However, Edd makes Eddy apologize to Rolf by presenting him with a potted plant. Unbeknownst to Eddy, giving a potted plant to someone is considered disrespectful in Rolf's country, and Rolf challenges Eddy to a fish duel. The duel takes place in a pit that Rolf dug himself on a tree that Rolf knocked over purposely in the process of digging the pit.
The DuelKevin pulls out a video camera to record the fight for his own pleasure. When the duel begins, Eddy doesn't put up much of a fight, and tries to back out of it by apologizing to Rolf for insulting him. However, he cannot express it without "The Cupcakes of Sorriness," which Ed had (inadvertently) originally suggested. After Eddy admits he does not have the cupcakes, he is savagely attacked and is hit at least eight times by Rolf swinging his Fish at him. After Eddy falls, Rolf says that in order for it to be a fair fight, Eddy must attempt to whack him. Eddy tries to hit Rolf in a feeble fashion, but Rolf throws an almighty whack from his fish at Eddy, who falls into the pit in slow motion, finishing the duel.
Rolf then gives the Eels of Forgiveness to the Eds which makes Ed turn into a fish mutant for an unexplained reason, possibly an allergy to eels. Kevin is offered to join, but he declines. Rolf gets angry because he denied the eels and starts marching towards Kevin. The fish used in the duel is seen again as a part of Edd's time capsule in "The Good Ol' Ed." Eddy later references the eels of forgiveness when he says "What's Wolfgang McHairyback gonna do? Stick eels down our pants again?" in "No Speak Da Ed."