Iron is a basic element for all plant and animal cells, and in fact every living creature needs iron for metabolism and important enzymatically controlled processes. This trace element is involved in so many processes that they cannot all be listed here.

What’s this:

Iron is the basic element in reef aquaria and very important for a healthy reef tank. In natural seawater, iron is limited and with a concentration of only a few nanograms ler liter hardly detectable. The source for corals is therefore bacteria, phytoplankton and food. In most reefs, iron is the limiting factor and not, as is often read, the nutrients PO₄³⁻ and NO3-.


Too high iron concentrations have an effect comparable to over-fertilization. Corals are adjusted to a limitation of iron and cannot cope with permanently too high values. Iron itself is not stable in seawater and, like manganese, precipitates quickly. The use of zeolites in an aquarium system enhances the export of iron, so those tanks may require an additional iron dosing. The first visible effect of a moderate iron supplementation is the improving effect of green coral colour. Almost all colours benefit from a moderate iron supplementation. However, too high iron loads can easily lead to undesirable darkening of corals and promote algae growth. Iron is also involved in the availability of nitrogen. If there is a nickel and zinc deficiency in addition to iron, corals and biofilms can absorb or process nitrogen compounds poorly or not at all. As a rule, the nutrients in the system then increase massively.


In the Balling Light System, iron is removed from the system via biopolymers, and additional filtering via zeolites increases the effect. If the values are too low, iron in bioactive form can be added via the supply system or individual dosing

Indicator species:

Green and yellow corals are the best indicators of iron supply. Acropra tumida / yongei („Enzmann-Acropora“) shows a bright green colour with well adjusted iron supply. If the values are too low, green changes to pale yellow or light green. Hydnophora and many other LPS show their polyps only with sufficient iron supply. Most corals become pale when the iron supply is too low. But also with too high values, many LPS corals cannot be maintained. Overall the aquarium then looks pale and impoverished.

Value too high:

Control the salt mix (specification of the anti-caking agents), reduction of the addition dosage, partial water change, avoid values above 5 µg/l.(0,26 Filtering via Zeolight, activated carbon CarbL or Phos. Increased skimming and ozonization also help.

Value too low:

Dosing Elementals Trace FE, Organic, Balling Light Trace 2.

Variety heavy metal, nutrient salt
Benefit Yes essential
Reference value 0,1–0,2 µg/l (0,26
Skill Level orange, lowest dosages are sufficient
Source salt, supply systems, trace element mixtures, feed, Artemia
Available Elementals FE, Balling-Light-System, Feed, Organic, Food Energizer
Importance 1–6 5
Detection quality safe/relative

Balling Light:

In the Balling Light system, the iron is dosed via the Balling Light Trace 2. Values between 0.1–0.3 µg/l (0,26 are sufficiently high. In aquaria with regular partial water changes, higher Fe values tend to appear in ICP analyses, as the anti-caking agents used in the salt mix are also indicated. However, these have no biological relevance and are quickly exported from the system via skimmer. In our systems, iron is tracked by Color Elements Green, Balling Light Trace 2 and the product Organic.

Mostly, the iron input into an aquarium is too high. The iron is then deposited on the surfaces, and a depotsit is built up. Paradoxically, a high iron deposition may be present on the rock decoration, while little or no iron is detectable in water. This is due to the fact that when calcium and phosphate precipitate, iron can also be bound, which is then placed in the depotsits. The use of filter media such as zeolite and aluminium-based phosphate adsorbers also reduce the iron concentration in the water. This is necessary to obtain bright yellow coral colourations.

Zeolite provides substrate for bacterial biofilms, and another benefit of this mineral is that it binds iron and other trace metals very well. The formation of the biofilms also consumes high trace substances, which later can be skimmed off with dead bacteria.

In addition to the amount of iron supplementation, it also depends on the form and oxidation stage in which the iron is added. ICP analyses always only show the element itself and not the chemical compound in which it is present. Many iron compounds are only biologically useful to a limited extent (e.g. anti-caking agents in sea salt). Additional iron is added to the aquarium via food and filter media, especially artemia and phosphate.

Due to the high oxidation capacity in seawater, iron is present as iron 2+ or iron 3+. This distinguishes iron strongly from other metals that do not have this variability. By changing the iron to a higher level, many chemical processes are controlled. It is therefore important to provide the aquarium with sufficient (not too much!) bivalent iron (iron 2+). This can also be done with the food alone, if it is ensured that it is actually absorbed.

Many organic molecules in the aquarium water bind iron and make it organically available or contribute to it being discharged via the skimmer. In our systems we ensure that not too much iron is built up by adding biopolymers. This ensures a long-term stable system and no undesirable growth of bacteria or algae. Depending on the consumption and nutrient concentration, however, it may be possible that a supplementation may be required. This will be carried out by the basic supply systems Balling Light, Color Elements and the feed. A general dosing recommendation cannot be given due to the very different consumptions in the individual aquaria. We have therefore designed our systems in a way that makes an iron limitation more likely than overdosing. While an excess of iron makes the corals darker and promotes algae growth, green and red corals lose their colours and become greyish when iron is deficient.


Iron is always limited in nature because it is difficult to keep in solution in seawater. It is therefore always important to feed corals sufficiently well because the food provides them with the necessary elements and minerals. They also need these to be able to supply the zooxanthellae sufficiently.