first, your ammonia is gonna spike from things dying and the like. They'll decompose first into ammonia. Then, the bacteria that feed off ammonia will start to grow in population, and convert it to nitrite. nitrites will rise, and then the bacteria that feed on nitrites will start to rise in population. Then they'll convert that to nitrates, which will start to rise. then nitirate bacteria start to rise in population. as each population rises, the levels of testable chemicals will go down. your system doesn't actually have no ammonia, nitrites, and nitrates, but rather the ammount being produced, matching the ammount being used up by the bacteria. that make sense?