Unlike nature, an aquarium is a closed environment. The fish live in their own waste, which creates Ammonia. You have to clean your aquarium once a week or there will be too much Ammonia, Nitrite or Nitrate and the fish might die.
Nitrogen Cycle Stages
It takes a month for the Nitrogen Cycle to finish.
Frist stage: The cycle begins when the fish are introduced to the aquarium. Their waste and leftover food are broken down into Ammonia.
Ammonia begins rising by the third day the fish are introduced to the aquarium. Ammonia is highly toxic to fish and any amount of Ammonia (NH3) is dangerous.
Second stage: During this stage the Nitrosomonas bacteria eat the Ammonia and make it into Nitrite. Nitrite is also highly toxic to fish.
Third stage: In the last stage of the cycle, Nitrobacter bacteria eat the Nitrite and make it into nitrate. Nitrates are not toxic in low to moderate levels. High levels of Nitrate are dangerous.
You can use a test kit to check you haven’t got too much Ammonia, Nitrite and Nitrate.