reefcalcs

NitrateNO₃

Nitrate is the end-product of the nitrogen cycle: ammonia from waste and food becomes nitrite via Nitrosomonas, then nitrate via Nitrobacter and Nitrospira. Marine fish tolerate it fine into the hundreds of ppm; coral does not. The "ultra low nutrient" fad of the 2010s pushed many reefs to undetectable nitrate and then watched them crash from dinoflagellates. Aim for measurable but low.

Target ranges

Beginner-safe2–20 ppm NO₃
Optimal (consensus, updated 2022)2–10 ppm NO₃
Triton target<10 ppm
Natural seawater (open ocean)~0.7 ppm as N ≈ 3.1 ppm as NO₃

Why it matters

Photosynthetic corals use nitrogen for tissue and zooxanthellae growth. At zero nitrate, zooxanthellae starve, the symbiont relationship breaks down, and coral pales or expels symbionts ("starvation bleaching"). At sustained high nitrate, algae outcompete coral for surfaces and calcification slows.

The 2–10 ppm range gives coral something to eat without giving nuisance algae enough fuel. Phosphate matters more than nitrate for algae growth, so a tank with 5 ppm NO₃ and 0.04 PO₄ usually looks better than one at 0 NO₃ / 0.03 PO₄.

Symptoms of drift

Too low

Below 1 ppm: ULNS conditions. Risk of zooxanthellae expulsion (starvation bleaching), dinoflagellate outbreaks, pale corals. The visual: SPS turn pastel, then white at the tips, then die back.

Too high

Above 20 ppm: nuisance algae proliferation (hair, bryopsis), depressed coral calcification at sustained >40 ppm, indicator of overfeeding or weak nutrient export. Fish are fine to 100+ ppm; coral health degrades earlier.

Testing

Hanna HI781 Nitrate Checker is the go-to colorimeter — 1 ppm resolution, 0–30 ppm range. Salifert Nitrate titration is acceptable but the color comparison is hard to read at low values. Red Sea NO₃ Pro works fine. ICP labs (ATI, Triton) report nitrate alongside everything else.

Test weekly. Reefs with carbon dosing (NoPoX, vinegar, vodka) need more frequent testing during initial dosing tuning; once stable, monthly is enough.

FAQ

Why is zero nitrate bad?
Coral need nitrogen and phosphorus to grow. Their zooxanthellae symbionts also need both. Driving nitrate to undetectable starves the symbiosis and shifts the tank's bacterial population toward problem species (dinoflagellates, cyano). Measurable-but-low is the modern target.
How do I lower nitrate?
Reduce feeding, increase skimmer wet skim, add chaeto in a refugium, carbon-dose (NoPoX/vodka — carefully), water changes. Address the input first; export tools only patch symptoms.

Test kits & supplies

Affiliate links

Salifert is reliable for hobby nitrate testing. For ULNS tanks fighting dinos, dose Brightwell NeoNitro to raise NO3 to 5\u201310 ppm.

Salifert Nitrate (NO₃) Profi Test kitColour-comparison nitrate kit that still resolves the low end reef tanks live in.View on Amazon →Brightwell NeoNitro nitrate supplementAdds nitrate to ULNS tanks fighting dinos.View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

Sources & references

  1. 1.
    Randy Holmes-Farley — Optimal Parameters (updated NO₃ range)
    https://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
  2. 2.
    University of Washington — Seawater Minerals
    https://depts.washington.edu/embryology/index.php?id=mineral
  3. 3.
    Randy Holmes-Farley — Phosphorus and reefs (Advanced Aquarist)
    https://www.advancedaquarist.com/2002/9/chemistry

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