Trace elements and metals

Here we are at chapter 4. After the chemistry, which I hope you enjoyed and did not find boring, we now enter the world of the infinitely small.

We are talking about elements we weigh in micrograms per litre, but that govern biochemical reactions. If the Triad (chapter 3) is the frame of the building, trace elements are the electrical wiring and heavy metals are the fuses: essential, but if overloaded we risk burning everything out.

In fishkeeping people often make the mistake of thinking: "if a little is good, a lot is better". Fortunately, lately this phrase is just a memory. With trace elements and metals it is the fastest way to turn a tank of Acropora into a graveyard.

In scientific papers, such as those by Sorokin and the latest publications in Nature (2025), trace elements are not seen as "colourants", but as enzyme cofactors.

I will dwell on the ones more or less everybody uses… 🤣

Element	Role	Value in the article	Ocean average
K · Potassium	the plumber	390-410 mg/l	399 mg/kg
Sr · Strontium	the reinforced concrete	8-10 mg/l	7.8 mg/kg
B · Boron	the pH thermostat	4-5 mg/l	4.5 mg/kg
I · Iodine	the anti-stress shield	0.06-0.08 mg/l	0.058 mg/kg
Mn · Manganese	the colour trigger	1-2 µg/l	0.02 µg/kg
Fe · Iron	the green pigmenter	0.1-0.5 µg/l	0.03 µg/kg
Zn · Zinc	the accelerator	1-3 µg/l	0.35 µg/kg
Co · Cobalt	the reef's vitamin	0.05-0.1 µg/l	0.0012 µg/kg
F · Fluorine	the invisible hardener	1.1-1.4 mg/l	1.3 mg/kg

Reference values from the article. If a little is good, a lot is not better.

Sources: [1]

The main trace elements, their role and the reference values. The last column, added by the editors, gives the average concentration in the oceans according to MBARI.

Potassium (K): the cell's plumber

What it is for: it is essential for moving nutrients and expelling waste from the coral's cells.

Ideal range: 390-410 mg/l.

Deficiency: growth tips that turn grey and look "burnt" or dry. The base tissue slowly starts to peel away.

Excess: hard to reach, but it can cause an unnatural darkening of the tissue and burnt tips.

Strontium (Sr): the reinforced concrete

What it is for: it is built into the skeleton together with calcium to make it dense and strong. It boosts the rate of calcification.

Ideal range: 8-10 mg/l.

Deficiency: the skeleton becomes as brittle as glass. Growth slows drastically even if calcium is perfect.

Excess: above 15 mg/l, growth stops and there is oxidative stress.

Boron (B): the pH thermostat

What it is for: it helps the coral keep the pH very high (about 9) in the space where it builds the skeleton. It is a biochemical "buffer".

Ideal range: 4-5 mg/l.

Deficiency: the coral uses too much energy to calcify and "lets go" of its colour. Growth stops.

Excess: above 7-8 mg/l it becomes toxic to the zooxanthellae, leading to slow fading.

Iodine (I): the anti-stress shield

What it is for: it acts as a powerful antioxidant. It neutralises the toxic oxygen produced by pushed photosynthesis (under the LEDs' blues).

Ideal range: 0.06-0.08 mg/l.

Deficiency: blue and purple colours turn dull or grey. The coral is more prone to bacterial infections.

Excess: above 0.1 mg/l, darkening of the tissue and too many zooxanthellae.

Manganese (Mn): the colour trigger

What it is for: it is the heart of the photosynthetic engine. Without it, light is not turned into energy efficiently.

Ideal range: 1-2 µg/l.

Deficiency: red and pink colours fade. The polyps of corals such as Goniopora stay closed or shrunken.

Excess: almost impossible in a tank, but if very high it favours filamentous algae.

Iron (Fe): the green pigmenter

What it is for: it is an essential cofactor for green proteins (GFP). It helps cell respiration.

Ideal range: 0.1-0.5 µg/l.

Deficiency: green Acropora turn yellow or pale. General loss of vitality.

Excess: it is nuisance algae's best friend and turns corals brown, because it makes the zooxanthellae population explode.

Zinc (Zn): the enzyme accelerator

What it is for: it is needed by the enzyme carbonic anhydrase, the one that lets the coral turn CO₂ into skeleton.

Ideal range: 1-3 µg/l.

Deficiency: calcification blocked. The coral seems "frozen" in time and does not grow a millimetre.

Excess: toxic. It causes the tissue to separate from the skeleton (polyps detaching).

Cobalt (Co): the reef's vitamin

What it is for: it is needed to make vitamin B12 and for the health of the bacterial flora (microbiome) that protects the coral's skin.

Ideal range: 0.05-0.1 µg/l.

Deficiency: the coral becomes vulnerable to every minor disease or parasite. Thin, fragile tissue.

Excess: toxic to coral metabolism if it builds up above 2 µg/l.

Fluorine (F): the invisible hardener

What it is for: it acts on the structure of the aragonite crystals, making them less soluble in acids (it protects the skeleton).

Ideal range: 1.1-1.4 mg/l.

Deficiency: a skeleton that tends to dissolve or weaken in shaded areas.

Excess: if too high it inhibits some photosynthetic processes.

Into the arena: ICP versus the reefkeeper's eye

Let's step into the arena like the gladiators 🤣 of modern reefing: ICP lab analysis versus the reefkeeper's eye. This is where the fate of the tank, and often of the wallet, is decided.

Some say "just look at the tank" and others will not move a millimetre without an ICP spreadsheet. As always the truth lies in the middle, but science and technology tell us exactly what we can see and what, unfortunately, is invisible to the human eye.

I have often asked myself: can we really "read" a deficiency, for example of vanadium, iron or boron, by looking at an Acropora? Or are we just "playing" at guessing? Let's see what molecular biology says and how to combine the two methods without becoming slaves to the test tubes.

The human eye is a fantastic detector… but careful: of trends. That is, over a period of time we can see whether the trend has changed or not. But it is a terrible analyst.

Often two factions form: the "eye purists" against the "ICP techies". We can compare it to driving a car: looking out of the windscreen or staring at the dashboard. If we ignore the windscreen we crash; if we ignore the dashboard we end up on foot because we have run out of petrol. Let's not form factions…

1. 1The eye · the windscreen sees trends and raises the alarm. But a fever does not tell you the cause.
2. 2ICP · the dashboard measures what the eye cannot see: the silent killers

Ignore the windscreen and you crash; ignore the dashboard and you end up on foot.

Windscreen and dashboard: the eye raises the alarm, ICP measures.

What does the human eye do?

It sees the coral's "face". If an Acropora closes its polyps, it is telling us something bothers it now. If the tissue thins, it is telling us it has been hungry for days. But a problem arises: "overlap".

And here, unfortunately, is the catch. A fading Acropora is like a man with a fever. The fever is the symptom, but the cause? It could be sunstroke (too much light), an infection (bacteria), poisoning (heavy metals) and so on.

By the time the eye notices that the colour is "less bright", the coral's internal biochemistry has been out of balance for a while. We are looking at the smoke, but half the house has already burnt down.

ICP is not a luxury

ICP-OES is the only way to see the silent killers. We can stare at an Acropora for hours, for days, but we will never understand whether that dull red is due to low manganese, a KH swing three days earlier or low boron. ICP removes the benefit of the doubt.

The case of Andrew Sandler (Polo Reef) is a textbook one: metals such as tin or copper can be present in traces that will not kill the coral tomorrow, but will block its growth and colour for months. The eye sees a "stalled" coral; ICP sees the poison holding it back.

The middle way

To be neither a "numbers fanatic" nor a "coral clairvoyant", the middle way is, in my opinion, the only winning one.

Let's use the eye for the alarm: if we see a change, we do not dose anything. We do the basic tests (KH, salinity, NO₃ and PO₄). If those are fine, we send off an ICP.

Let's use ICP for fine-tuning: every 2-3 months, to understand how much iodine, boron, manganese and so on the tank really uses. We do not supplement "by feel" hoping to see more blue: we supplement because the "dashboard" tells us we are on reserve, before we run out of petrol.

1. 1
   See a change?

   Do not dose anything.

2. 2
   Basic tests

   KH, salinity, NO₃ and PO₄.

3. 3
   If they are fine, send off an ICP

   Then one every 2-3 months.

ICP reads the total of an element, not how much is really available to the coral.

We are not afraid of the 10-day wait for the ICP. An Acropora does not die of vanadium deficiency in 10 days, but it does die from our hurry to correct something we have not measured.

The eye brings experience, ICP brings the cold truth. Without the eye the aquarium is a lab without a soul; without ICP it is a bet in which the stake is the life of our animals.

Careful: what an ICP really reads

ICP reads the total of an element, not necessarily the bioavailable part. ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometry) does not "see" molecules: it destroys everything it meets in order to read the individual atoms.

Nitrates: I often see ICP reports listing nitrates and KH, and reefkeepers saying "the ICP test told me my nitrates are high". False. Nitrates are compounds of nitrogen and oxygen, and ICP struggles to read nitrogen in the plasma. That is why almost no ICP test gives you real nitrates: if we find them in the report, they are often measured in the lab with other methods (photometry).

Phosphates (PO₄) versus phosphorus (P): this is crucial. ICP reads total phosphorus (P). The test we do at home (Hanna type) reads only inorganic phosphates, the ones immediately available. ICP reads everything: organic phosphates, those bound to proteins and inorganic ones. If at home we read PO₄ = 0.03 but the ICP tells us total phosphorus is very high, it means our tank is full of invisible "organic dirt" that sooner or later will turn into algae.

Iron: same story. The ICP will only tell us "you have 0.5 µg/l of iron". But in the tank we might have bioavailable iron or rust dust (toxic and useless). It is up to us to look and understand whether that iron is turning the corals green (good) or growing algae (bad).

ICP is an extremely powerful tool, but it is not an oracle (as in The Matrix). Without biological context and direct observation it remains only an incomplete chemical snapshot.

In short

Back to the famous statement: "I can tell iodine is missing from the colour of my Acropora".

As explained by Sorokin (Coral Reef Ecology) and confirmed by the data from Polo Reef (Andrew Sandler), by the time we see a colour change with the naked eye, the coral has already been biochemically deficient for weeks.

Doing an ICP test every 2-3 months is not "for fanatics": it is for smart people.

Sources cited by the author

• Nature Scientific Reports (2025). Comparative analysis of visual stress markers vs. ICP-OES trace element detection in Scleractinia.
• Skomal, G. (2020). Saltwater Aquariums For Dummies. (Section: Professional Lab Testing).
• Polo Reef (Andrew Sandler). Case study on heavy metal contamination in large reef systems.

Who wrote this article

Sources

1. Johnson K., Monterey Bay Aquarium Research Institute (MBARI) (2026). Periodic Table of Elements in the Ocean, Summary Table. MBARI.