Joker Fish are fish that've been subjected to Joker Venom, causing them to bear Joker's trademark grin. During "The Laughing Fish" storyline, he planned on copywrighting them, but was unable to since fish were a natural resource. Thus, he then planned on killing the patent clerk who told him he couldn't. Whoever eats a Joker Fish suffers the same symptoms as someone subjected to Joker Venom directly. They can also be used as clubs, as seen in "Batman: the Brave and the Bold" episode "Triumvirate of Terror".