Batman (Kevin Conroy) battles The Joker (Mark Hamill) in "The Laughing Fish", which is a loose adaptation of some classic Bat-mysteries involving the Clown Prince of Crime from the 70's. Harley Quinn (Arleen Sorkin) wasn't in the original story that introduced the Jokerfish, but Paul Dini's adaptation found room for her. Dini borrowed from not only Steve Englehart, who introduced the Jokerfish some 15 years earlier, but also "The Joker's 5-Way Revenge".
Joker doesn't really kill anyone in the TV version, as, rather, an anti-toxin is administered by Batman to each victim. The first such victim is a meek clerk named Francis......
Having read the original stories, I can honestly say Dini actually did justice.