The script had originally called for Elmo to morph into an electric eel and use his electric charge to light up underwater. However during pre-production it was realized that the charge from an electric eel doesn't actually generate light. Michael Schupbach suggested the idea of using an angler fish to executive producer Arlene Sherman.
Schupbach designed the character and the puppet was built by Rollie Krewson. Fred Buchholz built the light on his forehead to actually light up on cue; however, a post-production effect was used in the final cut rather than the working light.
- MichaelSchupbach.com - Portfolio Page on "Elmo Angler Fish"