Nitrification turns ammonia into nitrite then nitrate. Denitrification is the step that finishes the job: bacteria in low-oxygen zones use nitrate in place of oxygen and release harmless nitrogen gas (N2). Understanding it explains why some tanks hold nitrate steady for years and others climb steadily. Background: nitrogen cycle guide.
The chemistry in plain English
Heterotrophic denitrifying bacteria need three things: nitrate, an energy source (organic carbon, or for some specialists, sulfur) and little or no oxygen. With organic carbon, the overall reaction produces N2, CO2 and actually returns a little alkalinity. With elemental sulfur, autotrophic bacteria (e.g., Thiobacillus denitrificans) produce N2, sulfate and acid: 2 H2O + 5 S + 6 NO3⁻ → 3 N2 + 5 SO4²⁻ + 4 H⁺ (Randy Holmes-Farley, "When Do Calcium and Alkalinity Demand Not Exactly Balance?," Reef2Reef).
Where denitrification happens in a reef tank
| Site | How it works | Notes |
|---|---|---|
| Live rock interior | Oxygen falls off within millimetres to centimetres inside porous rock | Why mature rock "handles" nitrate better than new dry rock |
| Deep sand bed (DSB, 4–6 in / 10–15 cm fine sand) | Lower layers go anoxic | Effective but can trap detritus; don't disturb deep layers |
| Carbon dosing (vodka, vinegar, NOPOX-type products) | Mostly feeds bacteria that assimilate N and P into biomass, exported by the skimmer; some denitrification in biofilms | Needs a good skimmer; start tiny |
| Sulfur denitrator | Slow flow through sulfur media in a sealed reactor | Consumes alkalinity and lowers pH; effluent can smell of H₂S if flow is too low |
| Bio-pellets | Solid carbon polymer; bacteria grow on pellets, skimmer exports them | Similar outcome to liquid carbon dosing |
Sulfur denitrators and alkalinity
Because sulfur denitrification makes acid, it eats alkalinity. Holmes-Farley estimates removing 10 ppm nitrate per week this way uses about 0.3 dKH per week on top of the alk consumed when that nitrate was made from ammonia (Randy Holmes-Farley, "When Do Calcium and Alkalinity Demand Not Exactly Balance?," Reef2Reef). An aragonite chamber after the sulfur partly compensates, but it shifts the imbalance into extra calcium. Watch alk, pH and calcium if you run one, and use a 2-part so you can pause the calcium side if Ca drifts high.
Carbon dosing: watch the N:P ratio
Bacteria take up nitrogen and phosphorus roughly in the Redfield ratio (about 16 N : 1 P by atoms). If your phosphate is already near zero, carbon dosing stalls and nitrate stays high while phosphate bottoms out, which stresses corals. Test both before and during. More: nitrate and phosphate balance.
How to start carbon dosing safely
- Have a working skimmer (skimmer guide) and good oxygenation; bacterial blooms consume oxygen.
- Start at a fraction of the product's recommended dose and raise slowly over weeks.
- Test nitrate and phosphate 2–3 times a week.
- Stop or reduce if you see white bacterial film, cloudy water or both nutrients bottoming out.
Sources
FAQ
Does a deep sand bed really reduce nitrate? Yes, when built with fine sand and left undisturbed, the lower layers support denitrification. Coarse shallow beds mostly don't.
Why did my alk drop after starting a sulfur reactor? Sulfur denitrification produces acid, which consumes alkalinity. Increase alk dosing and monitor pH.
Is vodka dosing denitrification? Mostly not; it grows bacteria that take up nitrate and phosphate, which the skimmer removes. Some denitrification occurs in low-oxygen biofilm.