A new tank can be considered sterile in that the amount of life that is present in the water is very low to nonexistent. The first life in a new tank will usually be in the form of bacteria. The major problem with a new aquarium is that they have a dangerous "chicken before the egg" syndrome.
See you need something to produce ammonia before you can have ammonia-consuming bacteria. But at the same time ammonia is highly toxic to fish and inverts. So to get your tank where it can remove and process ammonia you first have to have ammonia. This usually results in deadly levels of ammonia being built up in your tank before the bacteria can catch up. Once the ammonia-consuming bacteria start consuming the ammonia in the tank they waste out nitrite. Nitrite is also toxic to fish and invert just like ammonia so you have another issue. To have nitrite-consuming bacteria you have to have nitrite in your tank. But this puts any live fish or inverts that would be in your tank in jeopardy as by this time they would have lived through potentially high levels of ammonia and now are suffering increasing levels of nitrite. As the nitrite bacteria grow they finally catch up with the nitrite production and the result is lowering levels of nitrite. Nitrite bacteria will then produce nitrate as their waste. Nitrate is not toxic to fish but is somewhat toxic to corals. One way of removing nitrate is by the use of a DSB (deep sand bed).
So to summarize the above paragraph you have to have ammonia before you can have ammonia-consuming bacteria. The ammonia bacteria will produce nitrite. The nitrite-consuming bacteria will then begin to grow only after there are nitrite levels in your tank. The nitrite bacteria will produce nitrate as a by-product. Nitrate is safe for fish but not for corals.