Iodine occurs in nature in various forms. Organic and inorganic forms are known, and there are many intermediate forms. For us aquarists, only the three inorganic forms of iodine are relevant: iodine and the oxidation forms iodide and iodate. The largest proportion of iodine in an aquarium is present as iodate, because even elementary iodine which is added to the aquarium water oxidizes to iodate in the alkaline and oxygen rich environment of seawater. UV radiation and ozone accelerate this process. Corals and biofilms can absorb all forms of iodine, so that it does not matter in which form it is added. All iodine compounds together result in about 60 µg/l. (0,26 In the aquarium the iodine value should be set between 60–80 µg/l. (0,26

What’s this:

Iodine is one of the most important trace elements for reef aquaria. Iodine occurs in nature in various forms. Organic and inorganic forms are known, and there are many intermediate forms. For us aquarists only the three inorganic forms of iodine are relevant: iodine and the oxidation forms iodide and iodate. Total iodine is measured via ICP measurements. Iodine is relatively stable in aquarium systems and is not consumed in the quantity that is often read.


A low iodine concentration shows up in dull tissue, reduced growth, colourless growth tips or growth edges and clear light sensitivity. Corals become more sensitive to parasites, dinoflagellate infestation by ejected zooxanthellae occurs more frequently. Too high values lead to darkening of the corals and increased algae growth.


Regular control of the iodine concentration and if necessary adjustment of the dosing quantity. Reduction by partial water change and adjustment of the dosing quantity. Iodine is the key element of the Balling Light Trace 3 solution.

Indicator species:

Acropora tenuis, blue or green Montipora species with blue margin. When total iodine deficiency starts, the green and blue colouring decreases, growth stops. If the total iodine deficiency increases, the sensitivity of corals to parasites such as AEFW (Acropora eating flatworms) and Montipora snails increases.

Value too high:

Reduce the addition dosage, partial water change, avoid values above 150 µg/l (0,26, filter via Carb L.

Value too low:

Dose Elementals Trace I, Color Elements red, increase basic dosing of Balling Trace 3.

Variety Halogens
Benefit 60-80 µg/l (0,26
Skill Level green
Source salt, supply systems, trace element mixtures
Available Elementals Trace I, Balling-Light-System, feed (algea)
Importance 1–6 6
Detection quality safe
Relation values PO₄³⁻, bromine, fluorine, carbonate hardness

Balling Light:

In the Balling Light system, the iodine is dosed as an iodine mixture via the Balling Light  Trace 3 solution. Values between 50 µg/l. (0,26 and 80 µg/l. (0,26 are sufficiently high. For SPS tanks and strong lighting, a value around 60-70 µg/l. (0,26 is recommended. You can also set this value directly via Elementals Trace I (Iodine). When dosing iodine, always ensure the correct ratio of bromine and fluorine.


A clearly visible sign of an iodine overdose is the increased growth of various algae in the system. Algae take up iodate and convert it into organic iodine and/or iodide. At high nutrient concentrations and high iodine values, the growth of algae increases dramatically. Most of them are green-brown to beige in colour and grow particularly fast on the glass surfaces of the aquarium.

Each of the different iodine forms is very independent in its chemistry and cannot simply compared to another. As already described, elementary iodine (I2) is also added with products based on Luglol’s solution. Iodine is a very strong disinfectant, the other two oxidation forms iodide and iodate, however, not at all. Iodine is also an inhibitor, but at present there is hardly any information available about the concentration above which the individual coral genera or even species react negatively to iodine, and in what way it can be distinguished. As with all other important values, the iodine concentration should correspond to other values such as carbonate hardness or PO4 concentration.

Iodine has a regulating effect in the aquarium. Especially organic compounds are used by corals and other creatures as repellent. At high doses, iodine is reduced to some extent by strong algae growth. Due to this fact, algae refugia seem to be a good thing, but as always the devil is in the details: algae can convert inorganic iodine into organic iodine compounds. However, these have a growth-inhibiting effect and are practically impossible to analyse.

With iodine, the consumption is high due to biotic and abiotic factors, so that each aquarium must find its own dosage. As algae release further growth-inhibiting substances and are also in competition with corals, we do not recommend refugia if stony corals are to be kept. Higher algae are rarely found in a reef. Their preferred habitat are other zones of the coastal areas.

An overdose of iodine shows up quickly, especially in SPS corals. At low nutrient levels, an overdose from about 80 µg/l. (0,26 leads to a decrease in colouration, up to a complete brown colouration of the corals. If yellow shades are desired and green fluorescence should not dominate, this value must be observed.

If there is an overdose or if one suspects that organic iodine compounds are present in too high concentration, they can be removed from the water by activated carbon and other adsorbers.

As in the field of macro elements, there are important relations to be considered in the field of halogens. An increased iodine value is often present at the same time as a too low fluorine value. If the iodine value in your analysis is too high, check your fluorine value immediately and adjust it to the reference value. In most cases the problem is already solved this way. You will find the correct ratios and relations under „relation values“ on your test, or the knowledge page online.


Below a value of 30 µg/l. (0,26, the situation becomes critical for many corals and other tank inhabitants, and we strongly advise not to go below this value. Since iodine is involved in an extremely large number of biological processes, it is one of the most important elements in reef aquaristics.

Iodine deficiency quickly becomes noticeable through lack of growth and greying of colours. The animals also appear to be more sensitive to light. Polyps are retracted early, algae grow slow or die off. At very low levels, dinoflagellate growth occurs and brownish-beige coatings develop on substrate and glass sheets. Also the nutrient degradation no longer runs smoothly. With LPS corals it can be seen that the animals do not expand to their maximum, even during feeding.

Overall the aquarium becomes colourless, dull and greyish. With extreme iodine deficiency, SPS corals lose their growth tips, increased shifting occurs and RTN-STN infections occur more frequently. With acute iodine deficiency, SPS corals shed off tissue from the base.

In connection to fluorine/bromine deficiency, a significant iodine deficiency is one of the main reasons for a dinoflagellate outbreak, especially under nutrient deficiency conditions. These are often zooxanthellae, which were released by corals under stress, or other toxic dinoflagellates take advantage of the lack of inhibitors and propagate strongly.

To compensate for an iodine deficiency, we recommend combination products of iodide and iodate. You can make up for this by using Balling Light Trace 3 or Color Elements Red directly and in relation to other elements. For individual supplementation and specific influencing of the values, we recommend Elementals Trace I (Iodine).


Corals can absorb all forms of iodine. Elemental iodine is quickly oxidized in the aquarium and is then present as iodide or iodate. The corals do not care how they get their iodine. In contrast to many other statements, iodine is quite stable in seawater. If you see a strong reaction on the animals after replacing the activated carbon, change to Carb L and activate the activated carbon and other filter media before use.