Would be interested to hear about the ammonia vs ammonium thing, although as we both know it's probably a bit advanced for me! But interested to try and learn, if you don't mind trying to explain chemistry to a brick wall
I am not a chemist, but I may be able to partially explain this, using some research I did for an article a few years back.
pH stands for
pondus hydrogeni, Latin for “potential of hydrogen.” Water is H2O, meaning two atoms of hydrogen and one of oxygen. Water actually "exists" as free positively-charged hydrogen ions and negatively-charged hydroxyl ions in varying proportions, and pH is the measurement of the ratio of hydrogen and hydroxyl ions in a body of water. Acidic water contains more hydrogen ions, and basic (alkaline) water more hydroxyl ions; neutral water has an equal proportion. The pH is closely linked with the level of carbon dioxide (CO2) because CO2 produces carbonic acid. The hardness also impacts pH, since the carbonates bind to acids as they appear; this buffering will prevent or limit changes in pH. The higher (greater) the KH (carbonate hardness or Alkalinity), the more "buffering" effect it has on pH.
The pH is measured with a scale from 1 to 14 with 7 being neutral. Numbers below 7 indicate acidic water, increasingly more acidic as they lower, while numbers above 7 indicate basic or alkaline water, increasingly as the numbers rise. This scale is logarithmic, meaning that each unit is a ten-fold increase/decrease; so a pH of 5 is ten times more acidic than a pH of 6, and 100 times more acidic than a pH of 7, and a thousand times more acidic than a pH of 8. Fish must never be exposed to sudden pH changes approaching one unit, as this is very stressful and may kill some species.
Nitrogen comprises about 80% of our atmosphere, and every life form on earth works hard to acquire it. In the aquarium, nitrogen exists in four/five forms: ammonia [NH3], ammonium [NH4], nitrite [NO2] and nitrate [NO3]. De-nitrification involves the fifth form, nitrogen gas.
Ammonia is a by-product of all aerobic metabolisms—fish, snails, invertebrates, fungi and bacteria; it naturally occurs from continuous biological processes and living organisms in any aquarium, and even at very low levels this ammonia is very highly toxic to all life. At levels between 0.5 and 1 ppm there can be long-term or permanent gill damage. Ammonia is never healthy at levels that can be detected by our standard test kits, and in most cases will have negative effects on the fish. Ammonia is produced by fish respiration; it is not present in urine, unlike mammals. It also occurs with decomposition of organics.
Ammonia dissolved in water rapidly associates to produce ammonium ions (NH4-) and hudroxyl ions (OH-). As the pH rises and temperature increases, so progressively more free ammonia is formed as these ions dissociate into ammonia and water(NH3 and H2O). Thus, ammonia levels become far more critical the higher the pH, and temperature has a similar effect. At pH 8.0, only 5% of the ammonia is free, while 20% is free at pH 9.0.
Ammonia is highly toxic to fish. In acidic water (pH below 7.0) ammonia automatically ionizes into ammonium which is basically harmless. This does not occur as much in basic water (pH above 7).
