THE DOSAGE OF IODINE IN REEF TANKS

In the following article, we will provide you with some interesting facts about iodine and its function in biological systems. We will also introduce some methods for supplying your saltwater aquarium with iodine and explain their advantages and disadvantages. The last section will then deal with the special features of measuring iodine in saltwater aquariums.

Chemically speaking, iodine is an element from the group of halogens. In addition to iodine, halogens include fluorine and bromine. These elements are generally very reactive, so that after iodine has been added to the aquarium, it reacts quickly with other elements and may therefore no longer be biologically available there.

Because of this behavior, it is difficult to detect iodine in the aquarium, and conventional test kits can only measure the concentrations of very specific forms of iodine. This can easily lead to an under- or oversupply of iodine.

General information about iodine

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

The most common form of iodine is the two ions iodide (I ^- ) and iodate (IO ₃ ^- ), which are formed when molecular iodine (I ² ) is dissolved in water. However, molecular iodine is generally difficult to dissolve in water. The natural concentration of iodine in the oceans is in the range of 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 applies, for example, to most macroalgae. In red, green and brown algae, organic iodine compounds act as antioxidants that bind harmful hydrogen peroxide. Hydrogen peroxide is produced there as an undesirable byproduct of photosynthesis and can damage 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 (=plant eaters) from grazing on the algae.

High concentrations of iodine can also be found in sea squirts ( Ascidiae ). Sea squirts need iodine primarily 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 have a cytotoxic effect, so that a possible function as a protection against feeding is discussed.

Crayfish and shrimps ( Decapods ) require iodine for growth and molting. In the absence of iodine, the animals' newly formed chitin shell does not harden completely after molting.

An additional dose of iodine has a particularly positive effect when caring for various soft corals, such as xenias and tube corals. High concentrations of the element can be found in the tissues of these animals. Gorgonians and black corals require iodine for the synthesis (= formation) of their skeleton, and the proportion of iodine contained in them can be up to 23%.

Soft corals such as Sarcophyton and Gorgonians benefit from a sufficient supply of iodine

When adding iodine to saltwater aquariums, a positive effect on the color of stony corals has often been reported. Iodine is said to particularly support the formation of blue and violet colors. However, there are currently no scientific publications that support these observations.

Dosage of iodine in saltwater aquariums

As with the addition of other elements, it is advisable to use ready-made solutions that are available commercially when dosing iodine. However, you should only start with 50% of the amount recommended by the manufacturer and slowly increase the dosage if necessary. You should also regularly monitor the dosage of iodine using an appropriate test kit. If the iodine concentration in your tank exceeds 0.06 mg/l, you can prevent an overdose by using activated carbon.

There are three possible iodine sources available for dosing iodine in a saltwater aquarium: (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. It also serves as a disinfectant.

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

Due to the fact that Lugol's solution contains various forms of iodine (I ^- , I ₂ , I ₃ ^- , I ₅ ), 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 means that a possible overdose can quickly cause damage to your aquarium inhabitants.

In principle, Lugol's solution is very suitable for supplying iodine to 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 iodine tincture. The iodine contained is chelated using 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 saltwater aquariums than Lugol's solution.
However, it cannot be measured or can only be measured with difficulty. Under certain circumstances, PVP iodine can accumulate in the aquarium. The use of UV light or ozone can lead to the destruction of the iodine complexes, causing large amounts of iodine to be released suddenly.
This is the main reason why you should avoid dosing with povidone-iodine if possible and instead use Lugol's solution or potassium iodide.

#3 Potassium Iodide

In contrast to Lugol's solution, potassium iodide solution has no disinfectant 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 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 supply achieved with it.

To make a potassium iodide solution, dissolve 5 g of potassium iodide (KI) in 100 ml of distilled water. 0.25 ml of the solution should be added to 100 liters of aquarium water every week. If you want to use the potassium iodide solution you have prepared to supply your 400-liter reef aquarium with iodine, for example, you need to add 1 milliliter of it once a week.

Determination of the concentration of iodine in the saltwater aquarium

As mentioned above, iodine is present in a variety of different compounds in saltwater aquariums. However, commercially available test kits only detect a portion of these compounds, which is why the iodine concentration you measure does not correspond to the amount of iodine actually 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 the most sense if the form of iodine recorded is the same form in which you add it to your tank.

However, this is not possible in most cases when using Lugol's solution. Therefore, you should not try to achieve a maximum iodine concentration of 0.05 to 0.06 mg/l through dosing, but aim for a value that is far below this concentration. 0.02 to 0.03 mg/l is recommended. When using a potassium iodide solution, the iodide added can be detected by most tests and the dosage can be controlled relatively well. However, always pay attention to the manufacturer's instructions for the test kit you are using! However, nothing is more important than observing the effects of an additional iodine dosage on your animals. If you notice increased algae growth or your corals turning brown, you should stop the dosage immediately and wait until things improve.