Ammonia poisoning is a common fish isease in new aquariums, especially when immediately stocked to full capacity. Ideally, the level of ammonia (NH3) and ammonium compounds (i.e. those containing NH4+) should be zero. Although trace amounts are generally harmless, they can still lead to problems over time. Understanding the nitrogen cycle is essential for the keeping of any aquatic life. The amount of ammonia present is usually accompanied by a rise in pH. As ammonia is a base, it is stabilized by alkaline water. It can cause damage to the gills at a level as small as 0.25 mg/L.