Trace elements in the reef aquarium — why they matter

When a new hobbyist starts reef keeping, attention naturally goes to the three main elements: alkalinity, calcium and magnesium. These rightly take priority — without them, hard coral skeletons cannot grow and water chemistry breaks down quickly. But reef water chemistry has a third layer that easily slips into the background: trace elements.

Natural seawater contains roughly 70 elements classified as trace elements. They represent only a tiny fraction of the total salt mass — and yet their deficiency can halt coral growth, dim colours and weaken animal resistance in ways that are not immediately connected to chemistry.

Why trace elements are needed

The simplest explanation is enzymes. Most of the vital metabolic reactions in corals and their Symbiodiniaceae zooxanthellae occur via enzymes — and many enzymes need a metallic cofactor to function. Iron is essential for photosynthesis. Manganese protects cells from oxidative stress. Molybdenum is involved in the nitrogen cycle. Iodine functions in the antioxidant system.

When any of these elements runs out, the metabolic machinery does not stop abruptly — it slows. The coral does not necessarily die, but growth slows, colours fade and tissue becomes more susceptible to disease. This is the characteristic signal of trace element deficiency: not a dramatic crash, but a quiet deterioration.

A second perspective is biology more broadly. Trace elements are not just a coral issue. The tank’s bacteria, archaea and other microbes need them for their own metabolism. A healthy microbial community is the foundation of the tank ecosystem — and it too depends on trace elements.

The most important trace elements — a brief introduction

Iodine (I)

Natural seawater approximately 50–60 µg/L. Fauna Marin reference value for the tank: 50–70 µg/L (slightly elevated is acceptable).

Iodine is one of the most discussed trace elements, and for good reason: it is consumed and lost rapidly in a closed system. Iodine is involved in coral antioxidant defence and is thought to influence several physiological processes, though the precise mechanisms are still partly under research. Iodine also varies more between tanks than many other trace elements — algae-based feeding raises it, ozonation depletes it. Levels below 40 µg/L have been associated with elevated RTN/STN risk.

Strontium (Sr)

Natural seawater approximately 8 mg/L. Fauna Marin reference value: ~8 mg/L — the target matches natural seawater directly.

Strontium behaves like calcium and is incorporated into coral skeletons alongside calcium carbonate. It is a group 2 alkaline earth metal that competes for the same intracellular binding sites as calcium and magnesium. Strontium levels often remain reasonable with quality salt water changes, but ICP data tells the truth.

Potassium (K)

Natural seawater approximately 380–400 mg/L. Fauna Marin reference value: ~380–420 mg/L.

Potassium is technically a macro-element, but it is often discussed in the context of trace elements because it is not part of the big three and is not tracked in routine measurements. Potassium consumption in the tank is slow — it does not precipitate strongly or bind readily — but its role is significant. It is related to cellular ion balance and there are hobbyist reports linking it to colour pigmentation. In practice potassium stays stable without active management if the water change programme is sound.

Boron (B)

Natural seawater approximately 4.5 mg/L. Fauna Marin reference value: ~4–6 mg/L.

Boron is an essential buffering agent for seawater: in its borate form it constitutes part of the pH buffering system. In corals, boron has been shown to be involved in skeletal formation, and its deficiency can slow calcification. Boron is one of those elements that often shows up as deficient in ICP results, because it is consumed as a byproduct of calcium reactors or Balling systems without being supplemented separately.

Iron (Fe)

Natural seawater offshore extremely low, typically 0.001–0.1 µg/L — but considerably higher near reef zones and coast due to sediment sources. Practical tank target: ~1–2 µg/L (Fauna Marin / Oceamo consensus), at a biologically more active level than open ocean but safe.

Iron is a key photosynthetic element — it is an essential electron carrier in the photosynthesis reaction and in chlorophyll formation. Iron’s special characteristic is that it precipitates rapidly in water and becomes biologically difficult to use. This means that a single large dose does not work as well as small, repeated additions. The most common sign of iron deficiency is weak green pigmentation.

Manganese (Mn)

Natural seawater ~0.01–0.1 µg/L. Tank reference value in the same order of magnitude — kept reasonable, not driven to zero or allowed to accumulate.

Manganese acts as an antioxidant (as cofactor of superoxide dismutase), participates in photosynthesis and is involved in coral skeletal formation. Manganese also has a practical side effect: it precipitates readily in the tank and drags other trace elements down with it — in other words, manganese precipitation can cause several other trace elements to disappear from the water column simultaneously. This is why choosing a low-manganese salt is sensible.

Molybdenum (Mo)

Natural seawater approximately 0.01 mg/L (~10 µg/L). Fauna Marin reference value: ~0.01 mg/L — target matches natural seawater directly.

Molybdenum is a cofactor for enzymes involved in the nitrogen cycle, particularly nitrate reductase. In other words, it is part of the process by which organisms convert nitrate into a usable form. The risk of molybdenum precipitating or being consumed is small in most tanks, but ICP data reveals if the level has drifted to zero.

How to dose trace elements — ICP first, no guessing

This is one of the most common mistakes in reef keeping: dosing trace elements based on rules of thumb before checking what is actually happening in the tank.

Trace elements are extremely varied. One tank may consume iodine rapidly due to continuous ozonation, while a neighbour’s tank stays perfectly stable without any additions. Molybdenum barely disappears in many tanks, but boron may systematically fall without the hobbyist noticing anything. Strontium, meanwhile, is partly self-replenishing through quality salt changes.

The stability principle applied to trace elements is simple: information first, action second. An ICP analysis reveals the actual levels — and only then is there a basis for dosing anything. Without data, dosing is as much a guess as it might lead to excessive accumulation as to correction.

ICP analytics are offered by several laboratories, and the services are divided into two techniques: ICP-OES (optical emission) and ICP-MS (mass spectrometry). ICP-MS is more sensitive and detects elements at concentrations below the OES detection limit — this is significant for trace elements, whose natural levels are in the µg/L range or below. Good practice is to choose a lab that clearly states the method used and detection limits, and to stay with the same lab between test runs for comparability.

Riuttareef recommends the first ICP test be done immediately after cycling is complete, before adding any animals. This is the only way to ensure the starting point is clean: the test reveals any impurities from glass, plumbing, source water, rock, equipment or salt, and it provides a clear baseline from which to track trace element development. If the baseline is good, animals can be added with confidence. If something is already off before any animals, the cause must be investigated before anything living is added. Ongoing recommended testing interval is every 4–6 weeks.

Base dosing on the ICP result, not on product package default instructions. If a trace element is within reference range, do not dose it. If it is below, raise it carefully and retest.

Trace elements leave through many pathways

It is useful to understand how trace elements disappear from the tank in order to properly calibrate dosing.

Most trace elements are removed through biological uptake — into the tissues of corals and other animals. Some precipitate (especially iron and manganese). The skimmer removes dissolved compounds. GAC (granular activated carbon) absorbs some trace elements. Ozone oxidises iodine and bromine in particular. Water changes dilute: they remove trace elements but also bring in new ones with the salt.

Two particular consumers that are often overlooked: macroalgae and phosphate removers. Both bind trace elements effectively and can deplete levels quickly without visible warning. If the tank has a refugium with macroalgae cultivation or phosphate remover media in use, trace element consumption is noticeably higher than without them — and the situation requires daily dosing to compensate for losses. This is one reason why ICP data is essential: the consumption profile is so tank-specific that no universally applicable dosing table simply exists — regardless of what product packages often imply.

Where to learn more

This article is intended as a first look at trace elements as a whole. Riuttareef has practice-level and deep-dive articles planned for individual trace elements — particularly iodine, iron, strontium and boron — going deeper into mechanisms, measurement and correction doses.

Next, also explore the alkalinity, calcium and magnesium articles, which cover the three main elements and the balance between them.

Sources

1. Peer-reviewed studies

• Ferrier-Pagès, C. et al. (2005). Effect of nutrient enrichment on growth and photosynthesis of the zooxanthellate coral Stylophora pistillata. Coral Reefs, 24(1), 46–57.
• Shick, J.M. & Dunlap, W.C. (2002). Mycosporine-like amino acids and related gadusols. Annual Review of Physiology, 64, 223–262.
• Raven, J.A. (1990). Predictions of Mn and Fe use efficiencies of phototrophic growth. New Phytologist, 116(1), 1–18.
• Allemand, D. et al. (2004). Biomineralisation in reef-building corals. Comptes Rendus Palevol, 3(6–7), 453–467.

2. Hobbyist literature and brand documentation

• Dank, K. & Southernland, A. (2024). The Complete ICP Walkthrough: Every Element Explained. Beyond the Reef Podcast. Oceamo.
• Zlements (2024). Hybrid 2 Part Dosing System — Technical Manual. zlements.com.
• Aslett, C.G. (2024). Teach a Person to Fish… SPS Academy Part VIII. reefranch.co.uk.
• Fauna Marin (2020). Knowledge Base: Iodine, Iron, Strontium, Potassium. faunamarin.de/en/knowledge-base/.

3. Books and textbooks

• Borneman, E.H. (2001). Aquarium Corals: Selection, Husbandry, and Natural History. TFH Publications / Microcosm.
• Sprung, J. & Delbeek, J.C. (1997). The Reef Aquarium, Vol. 2. Ricordea Publishing.
• Falkowski, P.G. & Raven, J.A. (2007). Aquatic Photosynthesis (2nd ed.). Princeton University Press.