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".
"The Laughing Fish" was adapted into an episode of Batman: The Animated Series with the same name (a major difference being that Harley Quinn is present) In this version, the aformentioned venomous qualities of the fish are absent. Also, in "The Batman Strikes!" #16 (based on the 2004 KidsWB cartoon), Batman notices a sketched out plan for the scheme from "The Laughing Fish" in Joker's hideout. Joker quickly snatched the plan, denying it was his (saying it "had Mr. Freeze written all over it").