Nitrates, phosphates and the Redfield ratio in the marine aquarium

Guide to the management of nutrients

Introduction

Nitrates (NO₃) and phosphates (PO₄) represent two of the most critical chemical parameters in the management of a marine aquarium. They are essential nutrients for all photosynthetic organisms, among them microalgae, zooxanthellae, macroalgae and cyanobacteria included, but they rapidly become harmful when they accumulate or are present in imbalance.

The balance between these nutrients is not only a question of absolute quantity. It is the ratio between them, and in particular the so-called Redfield ratio, that determines the stability of the system, the health of the corals and the prevention of unwanted proliferations (such as dinoflagellates or cyanobacteria).

This article explores, specifically:

• The biochemical role of nitrates and phosphates in a reef;
• The limits of tolerance in the various types of tanks (ULNS, SPS, mixed);
• The validity and the adaptation of the Redfield Ratio to artificial ecosystems.

The objective is not only to understand the numbers, but to transform them into instruments of active and aware control.

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The role of nitrates and phosphates in marine ecosystems

Nitrates: function and metabolism

Nitrates derive from the oxidation of the ammonia (NH₃/NH₄⁺) produced by fish and by the decomposition of organic matter. They are assimilated by algae and zooxanthellae for protein synthesis.

Typical values in nature: from 0.1 to 1.0 ppm.

Values in a reef aquarium: from 0.2 to 5.0 ppm, depending on the type of tank.

Phosphates: function and sources

Phosphorus (P) is an indispensable macronutrient for the formation of cell membranes (phospholipids), DNA, ATP. In the aquarium it comes from feeds, organic detritus, redox reactions.

Typical values in nature: 0.01 – 0.05 ppm.

Recommended values in a reef aquarium: 0.03 – 0.1 ppm.

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The ratio of Alfred Redfield

Scientific origin

The Redfield Ratio (1934), defined as C:N: P = 106:16:1, was observed in the tissues of marine phytoplankton and in the average composition of the ocean.

Carbon (C)	106
Nitrogen (N)	16
Phosphorus (P)	1

Expressed in simplified form for aquariums:

NO₃ : PO₄ = 16 : 1 (in moles) → ≈ 10 – 16 : 1 in ppm

When one talks about dissolved nutrients in marine aquariums, there is often a tendency to oversimplify a question that is instead tremendously complex. The presence, and above all the balancing, between nitrates (NO₃⁻) and phosphates (PO₄³⁻) plays a central role not only in the growth of algae or zooxanthellae, but also in the behaviour of the entire biological system, including the bacteria, the hard corals and even those microorganisms that one often notices too late, such as dinoflagellates or cyanobacteria.

Anyone who has dealt with reef tanks for years knows very well that the aquarium is not a sea in miniature, but an artificial microcosm, with dynamics entirely different from those of the ocean. This is why the famous Redfield ratio, 106:16:1 (C:N:P), must never be taken as a rigid rule. It arises from measurements on oceanic phytoplankton, in stable and scarcely contaminated waters. In a tank, instead, we have artificial lighting, filters, skimmers, variable food loads and absence of natural turnover.

That said, ignoring the Redfield completely is the opposite error. It serves as a reference, not as a dogma. When one notices an invasion of cyanobacteria or stunted growth of the corals, often the real culprit is not a value off the scale, but an unbalanced ratio between nutrients. For example, phosphate too low and nitrate present, or vice versa.

Now, a technical aside is needed: the bench tests we use in the aquarium (Hanna, Salifert, etc.) give us values in ppm, whereas the Redfield is expressed in moles. Converting with the correct molecular weights (NO₃⁻ ≈ 62 g/mol, PO₄³⁻ ≈ 95 g/mol), that famous 16:1 in moles translates into something between 10:1 and 16:1 in ppm, according to the sources and the type of salt used for the dosages. It is a calculation worth doing at least once, just to understand what we are talking about.

In ULNS tanks (like those who use Zeovit), nitrates and phosphates must remain ultra-low, but never absent. Some persist in keeping them at 0, but the corals show immediately that something is not right: they lose colour, they withdraw, they slow down their growth. It happens because the zooxanthellae struggle to produce energy without those minimal micronutrients. Even just a PO₄³⁻ stable at 0.02 ppm can make the difference.

Those who instead follow more balanced methods such as Triton have an advantage: they can use the Redfield as an indicator for adjusting the dosage, to check whether the refugium is working well or whether the macroalgal biomass needs a “boost”.

Another point often ignored: the tests can lie. Especially when they show zero values. In reality, the nutrients are there – it is just that they are absorbed so rapidly that they are not detected. Some end up in the biofilm, others remain trapped in the pores of the rock. When the SPS corals slowly fade without an evident cause, it is precisely in those cases that an ICP test can save the tank.

So, what should be done? The answer is: observe, measure and act with method. If the nitrates are too high, one can act with more skimming, use GFO for the phosphates, or reduce the organic load. If instead there is a deficiency, one can dose KNO₃ or diluted PO₄³⁻, perhaps together with a light administration of organic carbon (vodka, acetate, NOPOX, etc.). But everything must be done in small steps, because the swings are even more dangerous than the values out of range.

At the end of the day, the Redfield is an instrument, not an objective. It helps you to understand where you are going. It tells you whether the system is coherent. But it is up to you – with experience, patience and a bit of common sense – to decide how to use it. Because every tank is different. And numbers, on their own, are never enough.

Applicability in the marine aquarium

In reef aquariums, the Redfield is not always 100% valid. Closed systems, in fact, have:

• Variable organic loads
• Artificial photoperiods
• Mechanical and chemical filtration
• Absence of predators and natural turnover

But as a guiding reference, the Redfield remains fundamental:

• It helps to prevent chronic deficiencies or excesses
• It indicates when to intervene with organic carbon or supplements
• It reduces the probability of triggering blooms of cyanobacteria or dinoflagellates

Redfield modified for the aquarium hobby

Various sources recommend an operational Ratio between 10:1 and 16:1 in marine aquariums.

⚠ However, in the marine aquarium it is not always advisable to maintain this “pure” ratio, above all in ULNS systems or in those with strong lighting, where a lower phosphate is preferred in order to limit the algae. It is therefore useful as a reference, but it must be adapted to the context.

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Recommended guide values for different approaches

Method	Nitrates (ppm)	Phosphates (ppm)	Ratio NO₃⁻:PO₄³⁻
Berlin Method	5 – 15	0.05 – 0.1	50 – 150 : 1
ULNS (Zeovit, etc.)	0.1 – 2	0.01 – 0.03	10 – 30 : 1
Triton	2 – 5	0.02 – 0.08	~30 : 1
Balling Light	5 – 10	0.02 – 0.05	~50 : 1

📌 Note: an unbalanced ratio (e.g. nitrates present, phosphates absent) can slow down bacterial growth, increase stress in the corals and favour dinoflagellates.

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Risks of excess and deficiency

Excess of nitrates (>20 ppm):

• Rapid algal growth
• Washed-out colours in SPS corals
• Oxidative stress in the tissues

Deficiency of nitrates (<0.5 ppm):

• Arrest of coral growth
• Zooxanthellae less active → paler corals
• Dominant bacteria less efficient

Excess of phosphates (>0.1 ppm):

• Inhibition of calcification
• LPS and SPS corals with thin tissues
• Biofilm and stubborn algae

Deficiency of phosphates (<0.01 ppm):

• Arrest of coral growth
• Interruption of the symbiosis with zooxanthellae
• “Glassy” or translucent corals

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Strategies for management

In case of excess:

• Use anti-PO₄ resins (e.g. GFO or Lanthanides with controlled dosage)
• Improve the skimming
• Dilution through water changes
• Reduction of the organic load (fish, food)
• Installation of a refugium with macroalgae (e.g. Chaetomorpha)

In case of deficiency:

• Targeted dosing of NO₃⁻ (e.g. Potassium Nitrate, Sodium Nitrate)
• Dosing of PO₄³⁻ (e.g. diluted KH₂PO₄ solution)
• Increase of food for the corals (indirect nutrients)
• Temporary slowing down of the mechanical/bacterial filtration

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Practical case – SPS ULNS tank

400 L tank with Zeovit:

• PO₄³⁻ stable at 0.02 ppm
• NO₃⁻ raised from 0.3 to 1.5 ppm through KNO₃ dosing
• Colour of the corals improved in 10 days
• Dinoflagellates completely regressed

💡 In ULNS, the phosphate is often limited effectively, but the nitrate tends to fall to zero. In that case, it is better to raise it manually with measured dosages.

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Information Boxes

📌 Technical Advice – Ideal nutrient target

• SPS tank: NO₃ = 1–5 ppm | PO₄ = 0.03–0.08 ppm
• Mixed tank: NO₃ = 5–10 ppm | PO₄ = 0.05–0.1 ppm
• Soft/lps tank: NO₃ = 10–20 ppm | PO₄ = 0.08–0.15 ppm

📌 Useful instruments

• Hanna Checker digital photometers
• ICP tests (ATI, Fauna Marin, Aquaforest)
• Automatic dosing systems

📌 Signs of imbalance

• Corals closed without evident signs = suspected deficiency
• Green films on the rocks but PO₄ at 0 = distorted tests or intracellular accumulations

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Glossary

• NO₃⁻ (nitrates): final product of nitrification, source of nitrogen for algae and corals
• PO₄³⁻ (phosphates): source of phosphorus; in excess it can slow down calcification
• Redfield Ratio: ideal C:N:P proportion for the balanced growth of phytoplankton
• Refugium: separate section with macroalgae to absorb excess nutrients
• Lanthanum Chloride: precipitating agent for phosphates, to be used with care
• GFO (Granular Ferric Oxide): resin for absorbing phosphates
• C:N😛 – Original Redfield Ratio between Carbon, Nitrogen and Phosphorus
• ULNS – Ultra Low Nutrient System: systems with almost zero nutrients
• Carbon dosing – Controlled dosing of organic carbon in order to stimulate the denitrifying bacteria
• ICP test – High-precision analysis of the dissolved elements through spectrometry

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FAQ

📉 Must I always aim for phosphates close to zero?
No. Phosphates that are too low can slow down calcification, cause bleaching and block the metabolism of the corals. An ideal range varies from 0.02 to 0.08 ppm, according to the type of tank.

🔄 Can I use the Redfield values rigidly?
It is not necessary to replicate it to the letter. The Redfield ratio is a good starting point, but it must be adapted to the specific requirements of your system. The important thing is to maintain coherence between nitrogen and phosphorus.

💊 How to dose nitrates safely?
Use KNO₃ or NaNO₃ diluted in RO water, and dose slowly, starting with 0.1 ppm/day. Carry out regular tests in order to avoid peaks.

⚖ Why is my phosphate zero, but I still have algae?
Probable organic accumulation or phosphates trapped in the rock and substrate. The algae absorb the phosphate before it is detectable in the tests. Consider an ICP test for confirmation.

📈 Is it better to dose by hand or to use automatic systems?
If you have a stable load and little variability, manual dosing can be enough. For complex or sensitive tanks, automatic dosing with continuous monitoring is preferable.

⚠ How important is it to maintain a good NO₃⁻/PO₄³⁻ ratio?
Fundamental. A marked imbalance can favour opportunistic organisms such as dinoflagellates or cyanobacteria and cause deficiencies for the corals. It is essential to monitor both values and not only one.

🎨 Are colorimetric tests reliable?
Yes, for routine control. However, they present margins of error. For a more accurate overview (above all in ULNS), an ICP analysis every 1–2 months is recommended.

⛔ What happens if both values are too low?
The system enters an energy deficiency: the corals slow down or stop their growth, the zooxanthellae reduce their activity and toxic microorganisms such as dinoflagellates can develop.

💥 Can I correct the ratio by dosing only carbon?
Partially. The dosing of carbon (e.g. vodka, acetate, NOPOX) stimulates bacterial growth, but it does not replace a careful balancing of NO₃⁻ and PO₄³⁻. Without available nutrients, the carbon can cause bacterial collapses or critical deficiencies.

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