The purple fish first appears as one of the Fancy! customers that SpongeBob steals a Krabby Patty from. He later appears when the scene is filled with customers after Mr. Krabs says, "...and you brought customers."
Purple Fish:[to SpongeBob] Yes?
SpongeBob: Good afternoon, sir. We're selling chocolate bars.
[Patrick then gets close to the fish]
Purple Fish: Why is the tubby here staring at me?
Patrick:[stares at Purple Fish] Focusing.
Purple Fish: Back off, Jack! [Slams the door on Patrick's eyes]
Patrick: DOH! [Looks around] Nice place you have here!