Questions about LR & LS in new tank

Hi All,

I am studying up in preparation to start a 15 to 20 gallon marine tank
with live rock, live sand, corals, shrimp and a small fish. Also
planning to use an Eheim canister filter and protein skimmer (unless
of course I'm advised otherwise). I've read a bunch of posts here as
well as the FAQ's.

The guy at the LFS is very informative and patient. The tanks in his
shop look great - I think he knows what he's talking about, though I'm
new to marine tanks.

I've been reading about live sand and have this question: How can the
sand at the bottom of the sand bed remain alive? Assuming there is no
under gravel filtering (which I do not plan to use). Doesn't ammonia
and nitrite have to flow past this sand to keep the bacteria alive and
well? If not, then why doesn't just ordinary non-live sand become
live sand naturally by just sitting at the bottom of a tank?

Another question I have is (and I know this probably sounds naive
but)if I buy live rock from the store, from a tank that looks clean,
does it absolutely need to be cleaned before putting into my tank. If
it looks clean to begin with, what exactly do I clean off of it?

Thirdly, if I put live sand in a completely empty tank (well with only
water in it) how can I assure that the sand will remain alive before I
put anything else in it (whether it's live rock or fish)? For that
matter how can I be assured the sand is indeed live when I purchase
it?

In general, I'm concerned about the sequence and timing of putting
this stuff into a new tank.

Regareds,

-MB