Film on surface

A recent thread on alt.aquaria discussed a film forming on top of the water
after a while. One answer suggested that this was a protein film of oils etc
from waste food and from the keeper dippng his hand into the tank. The other
suggestion was that this was in fact a precipitate of calciunm carbonate. The
latter explanation seems to make more sense to me as I filled a tank a while
ago, merely to check for leaks, and, being a very lazy person left it for some
weeks, and the film deeveloped without of course any feeding having been done or
much ibn the way of hand dipping (I live in a very hard water area) If this film
is indeed CacO3 (?), surely water siphoned from beneath the surface should have
a markedly lower kH. This does not seem to be the case.
I am setting up a plant tank, and am aimng, as per the faq and advice on ths ng
and mailing list, for minimal surface disturbance. Does anyone know for certain
what this film is, and what if anything one should do about it?

My father once said to me:
"Son, there comes a time in every man's life when he has to make a stand."
I have just bought a new fish tank, so I guess my time has come.
I'm off to the shed.                            (Apologies to Stephen Fry)
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