One Piece - episode 32 - 18 Jul, 2000

Zoro is captured by the fishmen after Usopp and Johnny ditch him, and it's revealed that Nami was part of Arlong's crew the entire time. When a group of Fishmen attack Cocoyashi Village, Usopp comes to the rescue. (Source: Wikipedia) East Blue saga: Arlong Park arc: (2/14)