Ammonia - a toxic by product. Perhaps at this stage I should clarify what is meant by metabolic ammonia. This is ammonia produced in the fish's body as a result of breaking down amino acids for use as an energy source. This involves a process called deamination, taking place in the liver, during which the amine chemical is removed from amino acids. All animals produce metabolic ammonia but, as it is such a toxic compound, virtually all other animals, including humans, immediately convert it into a less toxic substance before it is excreted.
Humans convert ammonia into urea, which is passed out of the body as urine.
Fish 'don't bother' to convert ammonia, they simply excrete it continuously from their gills into the surrounding water. In a natural environment the immediate dilution by thousands of gallons of water would render it harmless. Understanding the chemistry is not important here, it is enough simply to realise that these processes are taking place and if we want to maintain optimal water quality we need to consider the removal or neutralisation of all of these pollutants, not just ammonia. A koi pond has at least three major sources of health-affecting pollution.