Your lfs is wrong. When water evaporates everything but pure H2O stays in the tank including nitrates, nitrites, ammonia, and salt. As your tank water level goes down, the rest of the levels will go up (same levels, and less water = higher concentration). I think what he might have meant was that after time you don't have to do water changes just top it off. Alot of us are finding out that when your tank matures, your levels pretty much set themselves constant, and water changes are no longer necessary.