Chapter 14 / UNDERSTANDING LIVING WATER
Conversion and removal are different jobs.
At a glance
Nitrogen enters through food, organic debris, source water and other inputs. Decomposition and excretion can produce ammonia. In oxygenated habitats, microbes can convert it through nitrite into nitrate. This sequence is useful, especially where fish produce a continuing waste load, but the nitrogen is still present.
Plants and algae can take up nitrogen as they grow. Some microbial processes can convert nitrate into nitrogen gas under suitable conditions. Harvesting plants and removing waste provide other export routes. A complete view follows nitrogen through all these places instead of stopping when ammonia falls.
Ammonia also changes chemical form. The proportion present as un-ionised NH₃ rises with pH and temperature. That is why an ammonia result should be read alongside both measurements. A stable total-ammonia number can mean different exposure in cool morning water and warm, high-pH afternoon water.
The main idea: A fall in ammonia with a rise in nitrate is consistent with conversion, not total nitrogen removal.
In practice
- Record exactly what the test reports: total ammonia, ammonia as nitrogen, nitrite as nitrogen or the full ion. Keep the original units.
- When investigating a change, measure pH and temperature with ammonia. Look at feeding, oxygen, alkalinity, filter flow and recent cleaning together.
- Follow the trend through ammonia, nitrite and nitrate rather than interpreting one colour result in isolation.
Going deeper · the science behind this chapter
The overall ideal nitrification reaction is NH₄⁺ + 2 O₂ → NO₃⁻ + 2 H⁺ + H₂O. Oxidising 1 g ammonium-N requires about 4.57 g oxygen and consumes about 7.14 g alkalinity expressed as CaCO₃ on this reaction basis. Actual system balances also include growth and other reactions. [S06]
Ammonia oxidation involves different microbial groups, including bacteria and archaea. Some Nitrospira can perform complete ammonia oxidation, called comammox; other members perform nitrite oxidation. This diversity refines the simple two-step explanation without changing the need for suitable substrates and conditions. [S07, S08]
For dilute fresh water, the un-ionised fraction can be estimated from fNH₃ = 1/(1 + 10^(pKa − pH)), with pKa = 0.09018 + 2729.92/T and T in kelvin. At 25°C, the fraction is approximately 5.4% at pH 8 and 36% at pH 9. The relationship is temperature-dependent and needs additional consideration at higher ionic strength. [S05]
If TAN is reported as mg N/L, multiplying by that fraction gives NH₃-N. Multiplying again by 17/14 expresses the mass as NH₃. Keep the convention explicit whenever comparing measurements or interpreting biological effects.
Organic nitrogen → ammonia → nitrite → nitrate → uptake or gaseous loss.
Related guides
- The Role of Good and Bad Bacteria in Your Garden Pond: A Comprehensive Guide
- Pond Filtration Systems: How to Choose, Size and Maintain the Right Filter
Sources and further reading
- S05 · WHO/IPCS. Environmental Health Criteria 54: Ammonia (1986). Includes the Emerson temperature relationship.
- S06 · US EPA. Municipal Nutrient Removal Technologies Reference Document (2008). Stoichiometric nitrification and denitrification demands.
- S07 · Daims et al. Complete nitrification by Nitrospira bacteria. Nature (2015).
- S08 · van Kessel et al. Complete nitrification by a single microorganism. Nature (2015).
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