Ammonia(NH3) is a very toxic substance released by fish and other living organisms as a by product of the digestion process, and the constant die off and re-growth of various organisms found within the confines of our system.
Usually this is nearly immediately broken down and oxidized into Nitrite by nitrosomons. A bacteria that lives in/on and around our aquarium as part of the nitrogen cycle.
Levels of NH3 within the aquarium should always be undetectable on most test kits and even on the very best ones it should only be a very small reading below 0.01ppm. Higher readings are indicative of an immature or cycling tank, overfeeding, or that something has died causing a build up faster than the filtration system can cope.
I will discuss the whole nitrogen cycle on a later post.
source: andy hipkiss basic reef chemistry, fishlore.com, various others.
