Selenium is an essential trace element which can have a toxic effect in moderately high concentrations.

The concentration in natural seawater is around 0.15 µg/l 1– we therefore classify selenium as an ultra-trace element. Selenium occurs as selenite and selenate ions, as well as organically bound (e.g. in proteins) in natural seawater.2

It is interesting to note that a proteinogenic amino acid (i.e. an amino acid that is essential for building many proteins) also contains selenium, selenocysteine. Selenium-containing proteins have also been detected in Acropora millepora.3

Selenoccystein Molekül

Selenocysteine (and selenium in general) is often involved in redox reactions – it fends off oxidative stress and thus protects the cell from harmful influences.4 Although there are no significant scientific studies on this in corals, a protective function against light stress is obvious.

Selenium has so far received little attention in the aquarium hobby.

One reason for this is that standard ICP-OES analyses can only measure selenium in concentrations that are ~100 times higher than concentrations in natural seawater – and therefore already in the potentially toxic range.

Selenium was also one of the main reasons why Oceamo made ICP-MS analysis available for aquarists: Using ICP-MS, selenium can be reliably detected in the target range (and therefore well below 1 µg/l). As a result, the aquarium industry has generally paid more attention to selenium.

Method of analysis

Detection sensitivity Oceamo ICP-MS seawater analysis: very high sensitivity, detection limit in the range of 0.02 µg/l. Deficencies can also be reliably detected.

Detection strength Oceamo Classic laboratory analysis (ICP-OES): low, only significantly increased values can be determined

Our recommendation:

  • Oceamo Classic Seawater Analysis

  • Oceamo ICP-MS Seawater Analysis

Selen Messgenauigkeit im Oceamo Labor

Caption Text – Accuracy of Oceamo ICP-MS analysis for selenium

Measurement quality in the Oceamo laboratory:
Excellent accuracy and reproducibility in Oceamo ICP-MS seawater analysis (see graphs).

Method: ICP-MS
Mean accuracy @ 1 ppb: 101,66 (±3,36)% (9 Measurements split over 3 days)
Mean accuracy @ 5 ppb: 101,98 (±2,03)% (9 Measurements split over 3 days)
Mean accuracy @ 10 ppb: 102,50 (±1,84)% (9 Measurements split over 3 days)

Stability of selenium in the sample:
Over 8 days at room temperature or at -20°C no significant change in the selenium concentration in real samples was observed (see graph).

Caption Text – Sample stability selenium – Oceamo empirical value


We recommend a selenium concentration of 0.2-0.5 µg/l in the marine aquarium. Selenium is contained in Oceamo DUO KH, and also in Reactor O.

Never dose Selenium on the basis of a “not detectable” result of an ICP-OES analysis, as there is a risk of overdosing!

Potential problems

Signs of a selenium deficiency: Stagnant growth, especially with calcareous red algae. Stress symptoms by high illumination intensity, loss of zooxanthellae

Anzeichen einer Selen-Überdosierung: Gewebeverlust, ins. with SPS

Potentielle sources

The most common cause is overdosage.
However, selenium occasionally occurs as an impurity in (balling) salts or in food.



Potential problems


Potentielle sources


Literature/Source list:
1) Periodic table of elements in the ocean. (2024).
2) Wrench, J. J. Organic selenium in seawater: levels, origins and chemical forms. Mar. Chem. 12, 237 (1983).
3) Young, S. Characterization of selenium containing proteins in the coral Acropora millepora. 1–277 (2016) doi:10.25903/6W0G-FS82.
4) Stewart, M. S., Spallholz, J. E., Neldner, K. H. & Pence, B. C. Selenium compounds have disparate abilities to impose oxidative stress and induce apoptosis. Free Radic. Biol. Med. 26, 42–48 (1999).
5) Denk, C. Reaktormedien unter der ICP-MS Lupe. (2022).

You can increase your selenium levels with these products: