Beneficial bacterias digest Ammonia and create another toxic chemical known as Nitrite.
The Nitrite is then broken down by another beneficial bacteria which produces Nitrate which is a relatively harmless substance. During the cycle, beneficial bacterias absorb Ammonia as an energy source, and convert it into energy plus Nitrite. This process takes approximately ten days in most aquariums. Once the Ammonia concentration is low enough, other beneficial bacteria attack the Nitrite. The process of bringing the Nitrite to a safe level takes approximately six weeks.