It really depends on where you are in the life cycle of your aquarium. I fyou are just starting up, then the presence of ammonia and nitrite are part of the normal cycle that will lead eventually to accumulating nitrates, and no ammonia or nitrites. On the other hand, if your tank is mature, then something has died in the tank and is producing nitrogenous waste faster than your biofiltration can deal with it, and you need to start making water changes to help out.