Dosing Iodine in reef tanks

With the following article we provide you with some interesting facts about iodine and its function in biological systems. In addition, some methods for supplying your reef aquarium with iodine are presented and their advantages and disadvantages are explained. The last section then deals with the special features of measuring iodine in saltwater aquariums.

From a chemical point of view, iodine is an element from the group of halogens. In addition to iodine, fluorine and bromine are halogens. These elements are usually very reactive, so that iodine reacts quickly with other elements after being dosed in the aquarium and may therefore no longer be biologically available there. Because of this behavior, the detection of iodine in the aquarium is difficult and with the conventional test kits only the concentrations of very specific forms of iodine can be measured. This can easily lead to an undersupply or oversupply of iodine.

General information about iodine

Iodine occurs in a variety of forms in natural seawater. These are both organic and inorganic iodine compounds. The iodine contained in organic compounds is always bound to a carbon atom. Examples of organic iodine compounds are iodomethyl, diiodomethane or iodobutane. These compounds can account for up to 40% of the total iodine content in coastal regions of the sea.

The most common forms of iodine are the two ions iodide (I-) and iodate (IO₃^-), which are formed when molecular iodine (I²) is dissolved in water. However, molecular iodine is usually difficult to dissolve in water. The natural concentration of iodine in the oceans ranges from 0.02 to 0.05 mg/l.

Biological function of iodine

High concentrations of iodine can be detected in the cells and tissues of some marine organisms.

This is true, for example, of most macroalgae. In red, green and brown algae, for example, organic iodine compounds act as antioxidants that bind harmful hydrogen peroxide. Hydrogen peroxide is formed there as an unwanted by-product of photosynthesis and can lead to damage of organic molecules due to its strong oxidative effect. Another function of iodine in macroalgae is protection against predators. Large amounts of iodide are stored, which ultimately leads to a strong metallic taste and prevents herbivores from grazing the algae.

In sea squirts (Ascidiae) high concentrations of iodine can also be found. Sea squirts need iodine mainly for the production of mucus in their gill apparatus. They are also able to synthesize iodine-containing amino acids and proteins, some of which function as hormones. In addition, some of these proteins exhibit a cytotoxic effect, so that again a possible function as antifeedant is discussed.

Crabs and shrimps (Decapoda) require iodine for growth and molting. In the absence of iodine, the newly formed chitinous shell of the animals does not harden completely after molting.

Especially in the care of different soft corals, such as xenia and tube corals, an additional dosage of iodine has a positive effect. High concentrations of the element can be found in the tissues of these animals. Gorgonians and black corals require iodine for the synthesis of their skeleton, whereby the proportion of iodine contained therein can be up to 23%.

Soft corals such as Sarcophyton and gorgonians benefit from an adequate iodine supply

The addition of iodine in marine aquariums has often been reported to have a positive effect on the coloration of stony corals. Iodine is said to support the formation of blue and violet colors in particular. However, there are currently no scientific publications that support these observations.

Dosing of iodine in marine aquarium

As with the addition of other elements, it is also advisable to use ready-to-use solutions available in stores for the dosing of iodine. However, you should only start with 50% of the amount recommended by the respective manufacturer and slowly increase the dosage as needed. You should also regularly monitor the iodine dosage using an appropriate test kit. If the iodine concentration in your tank exceeds a value of 0.06 mg/l, you can counteract an overdose by using activated carbon.

For the dosage of iodine in the marine aquarium you have three possible iodine sources at your disposal: (1) Lugol's solution, (2) povidone iodine, and (3) potassium iodide.

#1 Lugol's solution

This iodine-containing solution is actually used in medical diagnostics and analytics. Besides, it serves as a disinfectant.

Lugol's solution is an iodine-potassium iodide solution in a ratio of 1:2. Since molecular iodine (I₂) dissolves poorly in normal water, potassium iodide (KI) is added first to form free iodide (I-), which can now react with the molecular iodine to ultimately form polyiodide ions.

Due to the fact that Lugol's solution contains various forms (I^-, I₂, I₃^-, I₅) of iodine, it tends to be very suitable for supplying your aquarium with iodine. However, some of these compounds are very reactive, which explains the use of Lugol's solution as a disinfectant, which can quickly harm your aquarium inhabitants in the event of a possible overdose.

The Lugol's solution is in principle very well suited for the iodine supply of a saltwater aquarium. However, you should avoid overdosing in any case. Therefore, always dose the solution very conservatively.

#2 Povidone Iodine

Povidone or PVP iodine is also known as tincture of iodine. The contained iodine is chelated with the help of polyvinylpyrrolidone (PVP). Iodine is bound to the PVP molecule, making it no longer freely available and less reactive. The iodine contained in the tincture is gradually released. This actually makes PVP iodine more suitable for dosing in marine aquariums than Lugol's solution.
However, it can only be measured with difficulty. Under certain circumstances an accumulation of PVP-iodine in the aquarium may occur. The use of UV light or ozone can lead to the destruction of the iodine complexes, resulting in the sudden release of large amounts of iodine.
This is the main reason why you should avoid dosing povidone-iodine if possible and use Lugol's solution or potassium iodide.

#3 Potassium iodide

In contrast to Lugol’s solution, potassium iodide solution has no disinfecting effect and is used in medicine primarily in the field of radiation protection. The iodide (I-) released during the dissolution of potassium iodide does not react reactively and is therefore just as suitable for dosing in saltwater aquariums as PVP iodine. However, it can be measured very well, which is why the risk of accumulation or overdosing is extremely low. The only disadvantage compared to Lugol’s solution is the lower effectiveness of the iodine intake achieved with it. To make a potassium iodide solution, dissolve 5 g of potassium iodide in 100 ml of distilled water. Every week, 0.25 ml of the solution should be dosed per 100 liters of aquarium water. If you want to use the produced potassium iodide solution, for example to supply your 400 liter reef aquarium with iodine, you have to add 1 milliliter of it once a week.

Determination of the concentration of iodine in reef aquariums

As already mentioned, in marine aquariums iodine is present in a large number of different compounds. However, commercial test kits always detect only a part of these compounds, which is why the iodine concentration you measured does not correspond to the actual amount of iodine available.

Before determining your iodine concentration, you should therefore check which forms of iodine can be detected by your test kit. Of course, it makes most sense to detect the form of iodine in which you supply it to your tank.

When using Lugol's solution, however, this is not possible in most cases. Therefore you should not try to reach a maximum iodine concentration of 0.05 to 0.06 mg/l by dosing, but aim for a value far below this concentration. Recommended is 0.02 to 0.03 mg/l. When using a potassium iodide solution, the added iodide can be detected by most tests and the dosage can be controlled relatively well. Still, always observe the respective manufacturer's instructions of the test kit used! However, nothing is more important than observing the effects of additional iodine dosing on your animals. If increased algae growth or brown coloration of your corals develops, you should immediately stop the dosage and wait until an improvement occurs.