In this situation, there's only so much of the sketch that you can script, because the behavior of the fish is unpredictable. The improvisation here is wonderful: "It's brunch right now; it's gonna be lunch later." The sketch keeps going for about a minute after the script (apparently) comes to an end, and I love that.
Naming two of the fish Lyle and Talbot is a really obscure joke. Even 30 years ago, I don't know how many adults would have gotten it, let alone kids.
I didn't remember watching this one as a kid until Ernie said, "Look at how they move their mouths; they go mulm-mulm-mulm." It's strange how one little detail or turn of phrase like that can be the key to unlocking one's memories.