There are two types of beneficial bacteria in an aquarium; those that turn ammonia into nitrIte and others that convert nitrIte into nitrAte. Fish create ammonia, so starting off you'll have higher ammonia levels until a sufficient number of the A>NI bacteria grow. Once these are caught up they will convert the Ammonia and the levels in the tank will slowly return to 0. In so doing they have created a sudden surge in NI levels, and the NI>NA bacteria until this point haven't had sufficient "food" to multiply rapidly. Once the NI levels spike the NI>NA bacteria will establish and begin converting the excess NI, bringing its levels to 0 as well. Each bacterial takes a week or two to get established and convert all it's "food", and the NI>NA bacteria basically have to wait until the A>NI bacteria start doing their thing.
Usually you'll see your ammonia spike around day 10 and the nitrIte spike around a week later. With enough lights and CO2 injection, your plants will basically keep the nitrAte levels in check. However, you'll still need to do regular water changes for other toxins that also gradually build up.