Nitrate — the other half of the nutrient pair

Why nitrate is covered last

The first four articles in the main elements series dealt with elements that corals and the entire tank actively consume. The big three (alkalinity, calcium, magnesium) are skeletal building materials. Phosphate is a cellular biochemistry building block that corals absorb mainly as particles.

Nitrate is different. It is the last oxygen-rich end product of the nitrogen cycle, left in the water after microbes, algae, coral communities and fish have taken what they want from nitrogen. Measuring nitrate is more like measuring the tank’s residual nutrient load than measuring what a coral is actually using.

This is the article’s single most important insight, and it sets Riuttareef’s view apart from old-school recipes. In traditional SPS-focused thinking, nitrate was the enemy to be suppressed to zero. That approach has produced as many dead tanks as successes.

Nitrate is the other half of the nutrient pair — phosphate’s partner, which must always be read alongside it, never separately. This relationship drives the logic of the entire article.

What nitrate does in coral biology

Short answer: not very much, compared to other nitrogen forms.

Corals can use several forms of nitrogen — ammonium (NH₄⁺), urea, amino acids and nitrate. Of these, nitrate is energetically the most expensive: the coral must first reduce it back to ammonium via the enzyme nitrate reductase (which requires molybdenum as a cofactor, among others) before the nitrogen can be incorporated into amino acids and proteins. Other nitrogen forms take more direct routes.

Fauna Marin states this plainly on their nitrate page: “Corals prefer to take up nitrogen as ammonium, while nitrate must first be reduced before uptake.” This is the official position from one of Europe’s oldest reef brands, not speculation.

This does not mean nitrate is irrelevant — a coral uses whatever nitrogen is available, and a stable low nitrate level is part of a healthy tank. But it means that what matters to the hobbyist is total nitrogen metabolism, not just the nitrate reading.

A fascinating new discussion is emerging — direct dosing of ammonium and urea as an alternative to nitrate-based nutrient management. This is its own separate topic and is covered in the deep-dive article New approach — ammonium and urea as nitrogen sources (#12). This reference-level article stays focused on nitrate.

Reference values

Fauna Marin’s target range for nitrate is 0.5–2 mg/l, measured by ion chromatography. This is the internal optimum for the Balling Light system, where nitrogen flow is tightly managed and nutrients enter the system primarily via fish food.

In practice, different tank types have different tolerances:

The higher upper limit for softie/LPS tanks is intentional. Most softies and many LPS can thrive at 10–25 mg/l nitrate without visible harm, as long as phosphate is proportionally in range. Forcing it too low is often a bigger mistake than leaving it at a reasonable middle level.

A single reading is still meaningless without trend and context — more on this below.

Practical measurement and sources of error

Consumer tests on the European market:

• Salifert NO₃ Profi-Test — most reliable colour change at low concentrations, accuracy approximately 0.2–10 mg/l
• Red Sea NO₃ Pro — wide measurement range 0.125–32 mg/l, but colour interpretation requires good lighting
• Hanna HI-781 Checker — photometer, numerical reading; sensitive to calibration and sample temperature

One important source of error: the Hanna photometer produces false nitrate readings in the presence of silicate. If the tank uses silicate dosing as a dino countermeasure, use Salifert or Red Sea Pro instead.

The most accurate measurement comes from an ICP laboratory. Fauna Marin’s lab measures nitrate directly by ion chromatography — this does not suffer from nitrite cross-reaction, which can interfere with reduction-based titrimetric tests. An ICP package also provides the TNb value (total nitrogen bound), which covers all nitrogen forms together. This is the single best indicator of the tank’s total nitrogen load.

”Zero” is not zero

Consumer tests typically have a detection limit of 0.2 mg/l. If the reading shows zero, it means there is less nitrate than the test can detect — not that there is none at all. A reading of 0.5 mg/l stable week after week tells you that biology is consuming exactly as much as is coming in — that is a good state, not a cause for concern.

Trend is everything

A single measurement is an observation. Ten measurements are data. Measure regularly, record results and look at the trend:

• Stable 2 mg/l week after week = biology and feeding in balance
• Declining 5 → 3 → 2 → 1 → 0.5 mg/l = something is consuming nitrogen faster than it is entering
• Rising 1 → 3 → 6 → 10 mg/l = load is exceeding removal capacity

Dosing and management

The nutrient pair logic

Nitrate is never managed without phosphate. This is one of Riuttareef’s core rules. Fauna Marin’s guideline ratio is PO₄ : NO₃ ≈ 1 : 100 (e.g. 0.05 mg/l phosphate + 5 mg/l nitrate). The ratio itself is not sacred — absolute levels and their stability are more important.

A quick summary of how to read the parameters together:

• Both in range → nutrient pair in order
• NO₃ at zero, PO₄ in range → nitrogen missing, raise nitrogen
• NO₃ in range, PO₄ at zero → phosphorus missing, raise phosphorus (see phosphate article)
• Both at zero → the 0/0 trap (covered below)
• Both too high → reduce feeding or increase export

The 0/0 trap — when nitrate is too low

This is Riuttareef’s single most important safety note. The 0/0 trap refers to a situation where both nitrate and phosphate test at zero on consumer tests. This does not mean they are absent — it means biology is consuming everything incoming before the test can detect it. The tank is living in chronic nutrient starvation.

What follows: beneficial microbes starve, competition for nutrients collapses, and the path opens for dinoflagellates (Ostreopsis, Amphidinium, Coolia, Prorocentrum) and cyanobacteria — both survive at low concentrations because they can fix nitrogen from air.

This is why many aggressively nutrient-stripped tanks crash. The old-school target values of 1000–3000 L LPS tanks do not translate to a small mixed reef — the balance between biomass and nutrients in a small tank is considerably more sensitive.

Rule: Nitrate and phosphate must never be at zero simultaneously. The correction for a 0/0 situation is not “dose more nitrate” — it is restore the feeding rhythm and give the microbial community time to rebuild. Recovery can take weeks.

Raising nitrate — food first

If nitrate is consistently below 0.5 mg/l and phosphate is in range, the tank needs more nitrogen. Riuttareef’s priority is unambiguous:

First: feeding. Fish food delivers nitrogen in the right biological ratios — as proteins, from which fish excrete ammonium through their gills, and as particles from which corals take their share immediately. Practical step: increase the dose by 20–30 % for two weeks, measure before and after. In most cases this is sufficient.

Particularly effective is particulate food that stays in suspension for several hours — e.g. Reef-Roids, Nyos Goldpods, FM Coral Dust, FM Min S. Such food simultaneously serves fish, corals and free-water suspension microbes.

Second: commercial products as compensation when increasing feeding is not an option. EU-available:

• Tropic Marin Nitra-Bio — potassium nitrate in an organic matrix
• Fauna Marin Elementals N — functional nitrogen solution, part of the Balling Light system
• Nyos Nitrate+ — potassium-based nitrate solution
• ATI Essentials Nitrate — nitrate solution

Always start at a lower dose than the manufacturer recommends — most tanks need 30–50 % less than the recipe dose. Adjust weekly, not daily.

Lowering nitrate — step by step

When nitrate is consistently too high, correct it step by step starting with the gentlest measures:

1. Review feeding. Overfeeding causes high nitrate in 80 % of cases. Reduce the dose by 20–30 % and measure.
2. Water changes. Small weekly changes are better than large infrequent ones. A 25 % water change reduces nitrate by approximately 25 %.
3. Refugium / macroalgae. Increase Chaetomorpha mass in the refugium, provide light, harvest biomass regularly.
4. Carbon dosing. Dosing a carbon source accelerates bacterial nitrogen assimilation. Detailed coverage is in the dedicated carbon dosing article.

Sulphur reactors and other denitrification devices are worth considering only in very large systems (1000+ L). In modern-sized setups, a good skimmer + carbon dosing + porous rock handle the same work without a separate device.

The market also offers chemical “nitrate removers” (Brightwell NitratR, Seachem De*Nitrate, Rowa NitratR, polymer-based media). Riuttareef’s position is clear: these do not work reliably in a reef tank. Save your money and focus on steps 1–4.

Common problems and their causes

Closing — the stability principle in nutrient pair summary

The final article in the main elements series ends with the same principle as the four before it: trend tells more than a single number. This is especially true for nitrate, because a single reading is a transient snapshot of residual nutrient and says very little about what is actually happening to corals and the microbial community.

Three things to remember:

1. Nitrate is never managed without phosphate. The nutrient pair is always read together. A single parameter means nothing without the other.
2. Feeding is the primary tool for nutrient management, chemistry is secondary. The simplest fix for a 0/0 trap is giving the fish more food, not opening more expensive bottles. The simplest fix for high nitrate is feeding less, not buying a reactor.
3. Nitrate is a nutrient, not an enemy. A small stable concentration is a welcome part of a functioning tank. Zero should never be the goal.

“Measure both. Follow the trend. Feed first, then dose. One variable at a time.”

Sources

Peer-reviewed studies

• Grover, R., Maguer, J.-F., Allemand, D., & Ferrier-Pagès, C. (2003). Nitrate uptake in the scleractinian coral Stylophora pistillata. Limnology and Oceanography, 48(6), 2266–2274.
• Grover, R., Maguer, J.-F., Allemand, D., & Ferrier-Pagès, C. (2008). Uptake of dissolved free amino acids by the scleractinian coral Stylophora pistillata. Journal of Experimental Biology, 211, 860–865.
• Rädecker, N., Pogoreutz, C., Voolstra, C. R., Wiedenmann, J., & Wild, C. (2015). Nitrogen Cycling in Corals: the Key to Understanding Holobiont Functioning? Trends in Microbiology, 23(8), 490–497. DOI: 10.1016/j.tim.2015.03.008

Hobbyist literature and brand documentation

• Fauna Marin Knowledge Base — Nitrate/Nitrogen page. https://www.faunamarin.de/en/knowledge-base/nitrate-nitrogen/
• Tropic Marin Carbon Dosing system — product documentation and Lou Ekus podcast interviews
• Coral Garden Podcast — Dong Zhao double episode, nitrogen sources and coral biology
• CORAL Magazine — Cycling series Jan/Feb 2022

Books and textbooks

• Aslett, C. G. (2024). Holosystemics: Captive Reef Function versus Malfunction. Reef Ranch Publishing. https://www.reefranch.co.uk/