Ammonia buildup in the aquarium results from a breakdown of fish metabolism. Since ammonia (NH3) constantly converts to ammonium (NH4+) and vice versa, ammonia test kits usually measure both, resulting in a total ammonia (ammonia-N) concentration.
In an established tank, the reading of this test needs to show an undetectable level at all times. A detectable presence of total ammonia requires immediate action.
Ammonia is highly toxic in freshwater tanks, but even more toxic in reef and saltwater environments. This is due to a higher pH level that causes the presence of ammonia gas, which in turn is far more toxic and water soluble.
Even low concentrations of ammonia-N severely stress fish; this makes them vulnerable to diseases, thereby shorting their life span. Accumulating ammonia will not only be highly toxic and cause severe stress to the fish, but will be lethal!
Ammonia toxicity is significantly influenced by temperature and pH. The lower the temperature and pH, the more ammonia can be tolerated. For example, a fish tank kept at 68 degrees F can tolerate more than twice the amount of ammonia as a tank kept at 86 degrees F.
Lethal ammonia concentrations at a pH of 6.5 are 0.73 ppm, while at pH 8.5, only 0.17 ppm is considered lethal to inhabitants.