I have seen a sentence starting with "Es ist ein fisch", and I wondered why "es" is used with a masculine noun "fisch". I know that if something new is to be introduced, we can start with "Das" as in "Das ist ein mann" regardless of the gender of the noun "mann". But what I dont know is what's the point of using "Es" in such sentence? Does it have the same meaning if replaced with "Das"?
From a grammatical point of view, you probably can use both interchangeably. But there's a slight difference in the meaning:
"das ist" has something of an introduction or telling apart: "Das ist ein Frosch, (aber) das ist ein Fisch" or "Das ist Klaus". Think of pointing at the animal or person in question.
for "es ist" remember "It's a bird, it's a plane ... it's Superman" instead of "That's Superman". In German you wouldn't say "das ist Superman" either (except he's coming to tea and your parents wonder who that might be) but "es ist Superman" -- for want of a better term I'd call it explanation or recognition. The bell rings, everybody freezes in apprehension, but someone looks through the window and says (in relief) "Es ist Klaus!". A child is born and the midwife says "Es ist ein Mädchen." Everybody knows about the concept of boys and girls, so there's no point in "Das ist ein Mädchen", but presumably nobody knows what sex the newborn is, so the emphasis is on that information.
Come to think of it, it's probably the same pattern as in English (that vs it).