The nitrogen cycle: the biochemical engine of every aquarium

If you remember only one thing about aquarium chemistry, remember this: fish excrete a toxin, and two specific groups of bacteria convert it for you. Most beginner fish deaths and most veteran tank crashes trace back to a break in this chain. This guide treats it as a research brief, not a manual — what's actually happening, who the organisms are, and how to read the numbers.

1. The input: where ammonia comes from

Fish gills and kidneys continuously excrete nitrogenous waste, mostly as NH₃ (free ammonia) and NH₄⁺ (ammonium), the ratio set by pH. Uneaten food, decaying leaves, and dead microorganisms release additional ammonia through heterotrophic breakdown (the process limnologists call ammonification).

Only free NH₃ is acutely toxic. It is lipid-soluble, crosses gill epithelium directly, and disrupts central nervous system function. The charged NH₄⁺ is far less harmful because it cannot cross membranes passively. The ratio between the two is governed by pH and temperature: a one-unit increase in pH raises NH₃ concentration by roughly a factor of ten (Emerson et al., J Fish Res Board Can, 1975). The same 0.5 mg/L total ammonia is nearly harmless in a pH 6.5 blackwater tank and an acute toxicity dose in a pH 8.4 Rift Lake setup.

Practical consequence: aquarium ammonia test kits measure total ammonia nitrogen (TAN), not free NH₃. Reading the number without knowing pH systematically misjudges actual toxicity. The USEPA (1999) ammonia toxicity formula remains the standard reference.

2. The first oxidation: ammonia → nitrite

The textbook organism for this step is Nitrosomonas europaea. In aquariums, this is largely wrong. Hovanec & DeLong (Appl Environ Microbiol, 1996) sampled 17 freshwater ornamental aquaria using 16S rRNA probes and found that the dominant ammonia oxidizers were closer to Nitrosomonas marina and Nitrosospira clades, not N. europaea. This explains the inconsistent performance of bottled bacterial supplements built on N. europaea: the organism they contain is not the one that runs your filter.

More recent work shows that ammonia-oxidizing archaea (AOA, Thaumarchaeota) outcompete bacteria at low ammonia concentrations typical of stable, well-maintained tanks (Sauder et al., PLoS ONE, 2011; Bagchi et al., 2014). In mature aquariums, a meaningful fraction of ammonia oxidation may run on archaea rather than bacteria — one reason time-in-tank is not fully replaceable by additive products.

Nitrite (NO₂⁻) is also toxic. Absorbed across the gills, it binds hemoglobin to form methemoglobin, leaving fish unable to deliver oxygen even in well-oxygenated water — the classic "brown blood disease" (Tomasso et al., 1979).

3. The second oxidation: nitrite → nitrate

Textbooks attribute this step to Nitrobacter. Also wrong. Hovanec, Taylor, Blakis & DeLong (Appl Environ Microbiol, 1998), one of the most-cited papers in aquarium microbiology, used FISH probes to prove that nitrite oxidation in ornamental aquaria is almost entirely performed by Nitrospira moscoviensis-related organisms. Nitrospira grow slowly, have high affinity for nitrite, and are inhibited at high concentrations — which explains why nitrite spikes in new tanks linger so long, and why mature tanks show undetectable nitrite even under heavy bioload.

Daims et al. (Nature, 2015) identified a further surprise: comammox (complete ammonia oxidizer) Nitrospira capable of running the entire two-step oxidation in a single cell. Current evidence suggests these contribute meaningfully in low-load, stable filter biofilms — consistent with the long-observed "old tank syndrome" of resilience.

4. The terminus: where does nitrate go

Nitrate (NO₃⁻) is far less acutely toxic than its precursors, but chronic exposure above 40 mg/L impairs reproduction and immune function in many tropical species (Camargo et al., Environment International, 2005). Three removal pathways exist:

• Water changes: physical dilution, the most direct route. A 30% weekly change maintains nitrate at a steady state determined by feeding input and dilution rate.
• Plant uptake: aquatic plants strongly prefer ammonium (NH₄⁺) as a nitrogen source (Walstad, Ecology of the Planted Aquarium, 3rd ed., 2013, ch. 4), but consume nitrate when ammonium is unavailable. Densely planted stem tanks can sequester several mg/L of nitrate per week.
• Denitrification: in anaerobic microenvironments deep in the substrate or in densely packed media, denitrifying heterotrophs (often Pseudomonas species) reduce NO₃⁻ to N₂ gas. The pathway is minor in conventional tanks but measurable in deep-bed setups and marine sumps.

5. The signature of a healthy cycle

Two zeros, one moderate value, stable pH — that is the entire signature of a functioning nitrogen cycle.

6. Diagnostic logic for common failures

• Ammonia persistently elevated: media surface area is insufficient, or you recently rinsed off the biofilm. Hovanec emphasizes repeatedly that biofilter capacity scales with attached surface area and flow contact, not with the concentration of bacteria suspended in the water.
• Nitrite spike persists past two weeks: temperature too low (Nitrospira activity drops sharply below 22 °C, Lehtovirta-Morley et al., 2011), KH too low causing pH instability, or media packed densely enough to go anaerobic.
• Runaway nitrate: feeding exceeds removal capacity, or plants are in a stalled phase (low photosynthesis means low nitrogen assimilation).

7. What to actually do

1. Don't rinse filter media — and never in tap water. Chlorine kills your colonizing bacteria and resets the system to day zero.
2. Cycle the tank before adding fish. See the new-tank cycling protocol.
3. Testing beats dosing. Until you have data, every "water conditioner" is a guess.
4. To compute how NH₃ toxicity scales with your tank's pH, use the water chemistry calculator.

References

• Emerson, K., Russo, R. C., Lund, R. E., & Thurston, R. V. (1975). Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada, 32(12), 2379–2383.
• Hovanec, T. A., & DeLong, E. F. (1996). Comparative analysis of nitrifying bacteria associated with freshwater and marine aquaria. Applied and Environmental Microbiology, 62(8), 2888–2896.
• Hovanec, T. A., Taylor, L. T., Blakis, A., & DeLong, E. F. (1998). Nitrospira-like bacteria associated with nitrite oxidation in freshwater aquaria. Applied and Environmental Microbiology, 64(1), 258–264.
• Tomasso, J. R., Simco, B. A., & Davis, K. B. (1979). Chloride inhibition of nitrite-induced methemoglobinemia in channel catfish. Journal of the Fisheries Research Board of Canada, 36(10), 1141–1144.
• Daims, H., Lebedeva, E. V., Pjevac, P., Han, P., et al. (2015). Complete nitrification by Nitrospira bacteria. Nature, 528(7583), 504–509.
• Sauder, L. A., Engel, K., Stearns, J. C., et al. (2011). Aquarium nitrification revisited: Thaumarchaeota are the dominant ammonia oxidizers in freshwater aquariums. PLoS ONE, 6(8), e23281.
• Camargo, J. A., Alonso, A., & Salamanca, A. (2005). Nitrate toxicity to aquatic animals. Environment International, 31(7), 1075–1080.
• Walstad, D. (2013). Ecology of the Planted Aquarium (3rd ed.). Echinodorus Publishing.
• USEPA (1999). 1999 Update of ambient water quality criteria for ammonia. EPA-822-R-99-014.