"Where do you buy all those plants?"
"Wow!  An oyster!  I didn't know they were blue!"
Everyone's favorite is the snail on the glass, or the big aiptasia.  Sheesh.
I do think it's very funny that after you've just pointed out and explained your favorite parts of the tank- clownfish & host, coral REEF, etc, you then hear, "Is that saltwater?"
I think I knew about clownfish & anemones when I was 2, can you believe how many people have never heard of it, much less what an anemone looks like?