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What is Iodine?
Iodine is an essential element in many if not most organisms. It is
needed by humans and other vertebrates such as aquarium fishes mostly
to produce specific hormones e.g. thyroxine, which are needed to keep
the metabolism working. Invertebrates and algae have found many more
uses for this element, several of them use it as a kind of
Dosing iodine into a marine aquarium is a
controversially discussed topic. The purpose of this article is
to inform about what is known about iodine with regard to marine
organisms so far and when and how dosing may be adequate or
Iodine in natural sea water
The iodine content of natural seawater is roughly
between 0.02538 and 0.06345 ppm. The average concentration is somewhere
in between 0.050760 (Pilson, 1998) and 0,0584 (Millero, 1996). Iodine
occurs in several forms (think of them just as ammonia, nitrites and
nitrates are forms of the element nitrogen):
As an element (I2): this iodine
form is quite toxic and kills bacteria, but it is not very stable in
water and therefore not important with regard to natural sea water or
aquarium water (unless you add it, see below 'Lugol's
solution' and 'PVP-iodine'). 1 ppm of iodine can be lethal
to fishes, 0.2 ppm can kill Daphnia.
Iodide (I-): this form is far
less toxic and one of the two important inorganic forms in natural and
artificial seawater. Still, while fishes such as trouts can withstand
iodide concentrations of 100 ppm and more, surprisingly Daphnia
can be killed by 0.2 ppm of iodide in the water (Holmes-Farley, 2003),
indicating that overdosing can possibly have a negative effect on
invertebrates in the aquarium. A level of 0.2 ppm of iodide can be
reached in a crowded and well fed tank even without any specific iodine
Iodate (IO3- ): this
is the second inorganic form of iodine important in natural and
artificial seawater. In surface waters iodate generally has a higher
concentration than iodide (possibly due to a high ORP = oxygenation
reduction potential). Together, both forms can include up to 100% of
the available iodine. Lethal concentrations for Daphnia are in
the order of size of 10 ppm (unlikely to be reached) and for fishes
beyond 500 ppm.
Organic iodine (or organoiodines or organically bound iodine): This one is the big unknown with a large number of molecules involved, the most prominent ones being hormones, but also simple things like methyliodide (CH3I). Organically bound iodine can make up to 40% of the entire iodine in shallow marine environments. But this percentage as well as the exact composition will greatly vary depending on the specific body of water sampled.
Iodine in aquarium water
Shimek (2002a) found the iodine contents in tank water he examined to be at an average of 0.447 ppm with a standard variance of 0.518 ppm and a minimum concentration found being about 200% of the average in seawater. In all these tanks the iodine content was higher than in natural seawater. Interestingly, there was a correlation between high phosphate and high iodine measurements, which superficially can be interpreted as showing that both are added by too much import and too little export, most likely due to added foods.
Sherrill et al. (2004) found a relation of 0.18:0.14:0.11 for
iodate:iodide:organic iodine in artificial seawater at the Baltimore
aquarium, comprising between 0.43 and 0.48 ppm.
So, the average tank in the above studies had about
9x the iodine content of natural sea water. Both inorganic forms iodide
and iodate are found in sufficient concentrations at least in the water
column (this does not mean enough iodine in present in the
Iodine in organisms
Many marine organisms contain high or very high
concentrations of iodine. Compared to organisms not living in the ocean
or the iodine concentration in natural seawater anything above 0.1 ppm
could be considered high. However, in most cases it is not perfectly
clear if all the stored iodine is essential to the organisms or if at
least some of it simply taken up because it's available.
Algae, phytoplankton and Macroalgae
An extensive list of iodine contents in algae can
be found in Holmes-Farley (2003) showing there is quite a number of
algae, which take up and contain large amounts of iodine. Macroalgae
can take up iodate as well as iodide, but seem to prefer the latter.
Iodate can be turned into iodide using enzymes (Wong et al., 2002).
Macroalgae use iodide to avoid being overgrown by other organisms as
well as to adapt to lighting conditions (Balling, 2010). Having iodine
contents between about 50 ppm (or myg/g) up to more than several 1.000
ppm macroalgae are among the most important organisms with regard
to iodine. When talking about iodine containing food we have to talk
about algae. The iodine consists of mostly organic iodine and iodide.
Iodate constitutes only a small percentage.
Small list containing typical aquarium species (in ppm dry weight, Holmes-Farley, 2003):
Caulerpa taxifola 89
Caulerpa sertularoides 312
Caulerpa sp. in aquaria 440, 843, 1.083
Chaetomorpha linum 68
Corallina sp. 1.277
Gracilaria sp. 3.654
The iodine contents of Acropora tenuis corals (Zooxanthellae, tissue, skeleton) at One Tree Island and Magnetic Island are signifant with about 4-12 ppm (Reichelt & Orist, 2003).
However, aquarium specimens of Xenia sp.
and Sarcophyton sp. showed much higher concentrations of 135 and
95 ppm, respectively (Shimek, 2002b). Iodine can be used by corals to
support Zooxanthellae by improving their ability to adapt to new light
conditions (Delbeek & Sprung, 1996). A few words for those, who
hope to stop the decline of Xenia sp. colonies by adding iodine
(as often recommended): Go ahead and try it with reasonable doses (see
below), but don't have too high expectations.
Compared to gorgonians the iodine content in many
other marine organisms may seem negligible: several gorgonians have
contents of a few percent (remember: 1% is 10.000times 1 ppm) of their
dry weight making them even more important with regard to iodine
balance in a reef tank than macroalgae.
Small list of iodine contents of gorgonians
Eunicella otenocalloides 6.5 - 8.9% (= 65.000 -- 89.000 ppm)
Euplexora maghrebensis 0.19-0.23%
Gorgonia verrucosa 4.2-9.0%
Gorgonia lamarcki 3.3-6.8%
Gorgonia scirpearia 0.4-0.6%
Plexaura flexuosa 0.1 - 2.6% (= 1.000 -- 26.000 ppm)
Plexaura kukenthali 1.9-2.2%
Rhipidigorgia flabellum 0.6-1.1%
Antipatharian corals tend to overdo it with regard
to their iodine content, some contain 23% (230.000 ppm) of iodine, but
are of little importance with regard to aquarium care. From time to
time Cirripathes spp. are sold.
*Pics 3 and 4*
Another group of organisms, which contain significant amounts of iodine are sponges. Solimabi et al. (1981) found between 10 and 850 ppm in 12 different sponges.
Especially the shells of shrimps can have substantial iodine contents of up to 17 ppm, most of it being bound in organic forms (Haywood & Vilbrandt, 1931). This likely makes the shells a good iodine source for organisms, which eat it and shows that shrimps should not be feed without their shell. The flesh of shrimps contains significantly less iodine (about 1.3 ppm). If you ask yourself if your shrimps and crabs are getting enough iodine, just give them some dried (or fresh if they eat it) macroalgae such as Nori or Wakame. At least 30 ppm iodine are recommended for the diet of crustaceans such as Penaeus chinensis (Liu et al., 1995). It seems unclear if they can take up iodine from the water column, so better not count on that. Basically, this means many shrimps should be fed algae or algae containing food, because most other invertebrates as well as vertebrates do not contain that much iodine.
Marine bivalves typically have iodine contents similar to shrimp meat and fish around 1.3 ppm. As filter feeders this is possibly due to consumed phytoplankton.
To avoid iodine deficiency diseases in humans a diet containing marine fishes is generally recommended. Most marine fishes have iodine contents between 0.3 (herring) to 2.0 (pollock) ppm. Some have very high iodine contents, mullets up to 3.3 ppm and haddock up to 2.5 ppm. In general Gadiidae have high to very high contents for fish, while most flatfishes, salmonids and herring like fishes have lower contents. Even the marine fishes with 'low' iodine content have an order of size higher contents than most freshwater fishes (often < 0.05 ppm).
Several probable deficiency diseases observed in captive sharks, puffers, morays and other predators have been suggested to be caused by a lack of iodine, specifically growth or swelling of the jaw-neck-throat area similar to human goiter (feel free to look this term up to see the parallels) and lock jaw (when a fish apparently cannot open his jaws anymore).
The disc shaped thyroid gland of sharks and rays is found between the lower jaw and the gills. In contrast to that it is not a unified gland organ per se, but a spread number of cell conglomerates comprising the same function in most bony fishes. The majority of these cells is found in the throat-lower jaw area, too. If the fish diseases noted above are truly related to a lack of iodine is often unclear, however especially in some sharks the similarities to human goiter are striking.
Fish need iodine to produce the hormones tri-iodothyronine and thyroxine. If they lack these hormons thyroiditis, hyperplasia, adenoma, and adenocarcinoma have been observed (Sherill et al., 2004). Most marine foods fed to predatory fishes do contain sufficient amounts of iodine and should not lead to iodine deficiency at all. However, feeding only one or two types of food (e.g. krill, shrimps without shell) greatly increases the danger of deficiency diseases be it iodine or vitamin related. Therefore, as usual, I simply recommend a very varied diet in order to avoid deficiency diseases.
Other marine organisms
Two further organism groups are often noted when discussing iodine are tunicates and 'worms' (a rather inaccurate term including many unrelated groups of elongated invertebrates). The first ones seem to be similar to vertebrates with regard to the use of iodine contained in hormones, which is not a big surprise looking at their phylogeny. For the latter ones studies on typical aquarium species appear to be lacking, but there are at least some worms that do take up iodine by various means.
This paragraph lists the most important sources of
iodine in marine systems.
Tap water generally contains too little iodine to be relevant for an aquarium (that's one reason for approx. 2 billion humans with iodine deficiency). The marine salt mixtures we use to mix our water do contain iodine with unknown concentrations. It is assumed that these iodine concentrations are smaller than the concentration in the average aquarium (which is significantly supplied with iodine by food), but this is more a guess than a qualified statement and will likely also differ with the various brands. Also see 'Water changes' in the 'Export' section below.
In most aquarium systems food can be suggested to be the most important iodine source. Have a look at the iodine contents of some organisms listed above and you'll get an impression how much iodine would accumulate in our systems if there weren't some export on the one hand and biological processes adding iodine to organisms growing in the tank on the other hand.
Rocks, substrate, calcium reactor
Some rocks can contain significant concentrations of iodine of a few ppm. However, these rocks are not used as aquarium decoration. Live rock and dead live rock contain likely just as much iodine as the organisms settling on and in it. If they die or are eaten the iodine is set free. With regard to dissolving coral skeletons in a calcium reactor you can assume that up to about 10 ppm iodine are introduced into the system by the dissolving material.
For supplements containing iodine see the 'What to dose?' paragraph below. Depending on the doses administered supplements can make up a substantial contribution to total iodine of a tank.
This paragraph list the most important ways how
iodine is removed from a marine system.
Harvesting macroalgae does not only remove
nitrates, phosphates and a number of metals from the aquarium. Due to
their high iodine content of these algae, iodine can also be removed in
significant amounts. In fast growing algae filters or refugiums this
can be the by far most important way of phosphate
The same what is stated for macroalgae is true for
corals, especially for fast growing soft corals. Each cutting or frag
you give away exports some iodine from your tank. Each coral that dies
in your tank will increase the current
Balling (2008) doubts that partial water changes
alone are sufficient in providing enough iodine for an aquarium. To
extent this thought: typical water changes in a reef tank are likely
too small to have a significant impact on the overall iodine
concentration, but they can slightly shift the balance from organic
forms to inorganic forms (iodate, iodide).
It has been observed by a number of hobbyists that dosing a tank with iodine (mostly Lugol's solution or PVP-iodine) can result in an increase of skimmate. If this skimmate just consists of (organic) iodine removed from the tank can be doubted. It is more likely that this is also some bacteria (or protozoans) killed by the iodine.
Skimmate can contain significant concentrations of
iodine. Shimek (2002) found an average of 18.8 ppm in the skimmate and
130 ppm in the sludge. This may sound much, but considering the small
volumes of skimmate in most tanks it's not sufficient alone to get
the iodine concentration somewhere near the natural seawater level.
Filter cleaning (in those tanks that have some type of mechanical
filter) is another way of iodine export comparable to
Glaser (2008) suggested that iodine can be removed
from a tank by phosphate adsorbing media. However, numbers are lacking.
It can be expected that this effect is weak on low iodine
concentrations, especially if silicates and phosphates are present
(otherwise the adsorbing media would make little
Aside the processes of iodine import and export, which dictate the overall amount present in a tank, there are also numerous transitions of iodine from one form to another that predominantly can determine its availability to organisms, which after all is a very important factor for the hobbyist.
Iodate is the form of iodine for which the smallest demand (and therefore often the highest concentration) exists. Algae and phytoplankton can turn iodate into iodide with enzymes, but do apparently prefer the uptake of iodide for which they do not need energy consuming conversions. The conversion may actually be just an 'unwanted' byproduct of nitrate uptake. Iodate can be an accumulating 'endproduct' of biological and/or chemical processes (not unlike what nitrate is to nitrogen or phosphate for phosphorus). As noted above it's not too in demand by organisms, but can be used here and there, mostly to produce iodide. Oxidation processes (high ORP) can turn iodide into iodate while bacteria in low oxygen environments (live rock, sand bed) do the opposite (Farrenkopf et al., 1997) creating a kind of iodine cycle, which is not totally unlike the nitrogen cycle.
One big and often unknown factor within this
iodine cycle is organically bound iodine. Iodate as well as iodide can
be converted into various simple or complex organic molecules such as
iodinated amino acids, monoiodotyrosine, diiodotyrosine (Holmes-Farley,
2002) and humine complexes (Cook, 2000) to name only a few. The iodine
in such molecules can comprise significant amounts of the total iodine
depending on biological activity. Just have a look above on how much
iodine can be stored in a few gorgonians. Oxidation of organic iodines
likely will produce iodate again.
A few hobby test kits are available to test for iodine. First, there is the Iodide/Iodine test kit by Seachem. Iodate is apparently not detected by the kit (Holmes-Farley, 2003), and it is unclear if or which organically bound forms of iodine are included in the test.
The iodine test kit by Red Sea indicates that it can measure the total iodine content, which does give no information on the specification at all. Again, it's unclear if organic iodine is measured. A similar test kit is available by Aqua Light.
The Salifert iodine test kit in my opinion works well, the detection range is adequate. It measures iodine/iodate as well as iodide with two different test procedures. On the downside it is not clear if and which organic forms of iodine are measured.
Users of the various test kits above in part had mixed results.
Validating any test with a reference solution (preferably not home
mixed) with a given iodine concentration is a good
I have to agree with Holmes-Farley (2003) that
trying to achieve an iodine content similar to natural seawater by
supplementation and control with a hobby test kit is likely a
misleading approach, because many organic forms of iodine may not be
measured by the test kits. The consequence is overdosing. If you still
wish to go this way assume the iodine concentration in your tank is
twice or three times as high as measured (In part due to non-detected
organic forms, which may be true or not).
When not to dose?
- Typical Fish only (FO) or fish
only with live rock (FOWLR) setups with medium to high nitrate
(>10 ppm) and phosphate (> 0.1 ppm) concentrations. Due to the
typical amount of food used in these systems a lack of iodine is
- Mixed reef tanks, where everything is
running OK (yes, even the gorgonians), but you think
maybe iodine supplements could improve the situation a little bit. In
case of a lack of iodine I'd expect the gorgonians to show symptoms
first (Balling, 2010), while everyone else is still happy. Therefore,
I'd not dose any iodine supplement.
Please, don't be surprised that many or even
most marine tanks belong to the above two categories. There is a large
number of successful marine tanks in the last decades that never have
seen intentional iodine additions.
When to dose?
- Thyroidal growth: If
fishes such as sharks, groupers or morays show goiter like growth or
swelling only in their neck-throat-jaw area, I'd supplement the
food with iodine. What is more important is to change the diet to a
greater variety. You can even stuff some algae in between. Also check
(and if possible improve) your water quality and consider bacterial
infections, which can also lead to tumorous growth and swelling.
- In the case of lock jaw (an
often fatal condition), sometimes iodine supplementation of the water
is suggested. If you do (gently) try force feeding, add some iodide or
iodate to the food. If the fish survives re-evaluate and likely change
- Otherwise successful mixed reefs
with declining gorgonians and/or Erythropodium spp
(encrusting gorgonian). and/or star polyps (mostly Briraeum
spp.): Try Balling's (2010) recommendation given below ('How
much to dose'). Following him, especially Erythropodium can
be a good indicator organism for a lack of iodide.
- Large algae filters with slow
growth despite an otherwise adequate environment. If there is
enough of the standard nutrients nitrate and phosphate as well as
sufficient lighting, a lack of iron or iodine theoretically could limit
algae growth and adding a supplement might be worth a try. An
experiment described in Holmes-Farley (2003) did not show significant
increase of growth for Chaetomorpha and Caulerpa racemosa
with the addition of iodide in comparison to a control group without
iodide supplementation. However, the iodine background level of the
water in this experiment is not known and Caulerpa racemosa did
not seem to like the growing conditions - with or without iodide
supplementation - anyway. It has yet to be demonstrated that iodine
limitation is possible on the aquarium scale.
- Very low nutrient systems with little
feeding, strong illumination and only small water changes: In
these typical Acropora dominated SPS reef tanks a careful
supplementation of iodine can have minor positive results, though many
other factors such as illumination (strength, distribution and
spectrum), current and all the rest of the water chemistry certainly
have a much larger impact making the definitive detection of
improvement due to some supplement at least
- Very strong use of ozone: Use
of ozone can decrease both iodide and organic iodine to undetectable
and physiologically probably useless levels by turning it into less
available iodate (Sherill et al., 2004). This process can be more
efficient than the natural process of turning iodate into iodide (as
done by algae). It is one reason to limit ozone to a necessary
concentration. In part ozone can be replaced by other filter techniques
(conventional skimming, UV). If for whatever reason high levels of
ozone are used in a reef system (ORP > 400 mV) with low nutrient
levels or if by any other means such a high ORP occurs, iodine
supplements can be recommended. In ozonized FO and FOWLR systems iodine
rich foods (such as shrimps with shells and algae) should be fed. Given
very high iodine levels already available in a system, the conversion
from iodide to iodate can even be considered as a detoxification as
iodate is less toxic than iodide.
What to dose?
The aquarium and pharma industries offer quite a
number of iodine supplement products. Some aquarium industry related
companies don't feel like letting us know what form of iodine is in
their bottles at which concentration, so sometimes we have to guess and
consider if we trust the dosing recommendations given. Most iodine
supplements can be classified in a few categories:
- Lugol's solution is
typically made of 5 g iodine and 10 g potassium iodide (KI) mixed with
distilled water to a total volume of 100 ml and a iodine/iodide content
of 150 mg/ml. Typically it has a brown color and is used to treat small
- Iodine in complexes: The most
common complex is povidone-iodine (PVP-iodine, betaisodona). The iodine
is bound to organics and released with time. These complexes are more
stable than Lugol's solution and therefore can be preferred. It
still has antibacterial properties, though. The organic part of the
solution due to its small concentration is very unlikely to have any
negative impact on an aquarium.
- Potassium iodide: This
is one component of Lugol's solution. It is available as SSKI
("saturated solution of potassium ('K') iodide") or
as a crystalline powder. It is much less toxic than Lugol's
solution and has little antibacterial properties making it a good
supplement in general, but useless in treating wounds. USP SSKI
contains about 45-55 mg iodide per drop.
- Iodine as a part of general trace
element supplements: Many trace element supplements contain iodine.
How much exactly is generally not mentioned by the producer (let's
hope they had exactly your tank in mind when giving their
recommendations on how much to add'¦).
- Last not least: food.
As noted above, many if not most of the foods we use in marine tanks
contain significant amounts of iodine. Consequently simply increasing
the amount of food or mixing in some algae can help with any possible
iodine deficiency problem in tanks (where increasing the amount of food
is possible). This is the most natural iodine
Less common in aquarium use are Sodium iodide,
Calcium iodate, Potassium iodate and Sodium iodate. With the latter
being very stable, but apparently less likely taken up by organisms, at
Sometimes baths in iodine (Kugol's or
betaisodona)enriched water are recommended to fend of bacterial or
protozoan growth on coral frags. This topic is not covered by this
article, since the iodine is a simple disinfectant in this
How much to dose?
If you want to add iodine to your aquarium and cannot increase the food rations for whatever reason I recommend to use iodine in a complex (e.g. PVP-iodine) or SSKI. You can get it in a pharmacy or buy an aquarium product, the latter are sometimes even cheaper. Use daily additions if possible. Also, a test kit is actually not a bad idea if you consider dosing your tank. It may not be able to tell you how much iodine is bound organically, but it will give you at least a rough estimate on the rest. If the test kit shows already high numbers (about natural seawater or higher), I recommend not to dose any iodine. If the levels are more than 5x times higher than natural sea water concentrations better think of exporting some iodine.
When should I stop dosing?
- If the doses given above show no positive effect
within about 4-8 weeks I'd suggest to stop dosing your iodine
supplement and search for another reason for the problem you are trying
- If the doses given above show a definitive
positive effect, try to decrease the supplementation until this effect
disappears again, and carefully rise it again a little until its
re-occurrence. This is done to find the smallest necessary dose. It is
also slightly helpful to determine if the observed positive effect is
related to the iodine addition at all (a difficult task without a
- If the situation becomes worse, you can stop
dosing, because your problem is likely not a lack of
Balling, H.-W. (2010): Spurenelemente -- Fakten
und ZusammenhÃ¤nge (Trace elements -- facts and relations)-
Koralle 65, 32-41 (in German, there probably will be an English
Bingman, Craig (1997):The Halogens -- Part III:
Iodine.- Aquarium Frontiers. December 1997.
Brockmann, Dieter (2007): Jod - mehr Fragen als
Antworten (Iodine -- more questions than answers).- Koralle 42, 68-74
Cook, P.L.M. (2000): Speciation of dissolved
iodine in the waters of a humic-rich estuary.- Marine chemistry 69,
Farrenkopf, A.M. et al. (1997): Reduction of
iodate in seawater during Arabian Sea shipboard incubations and in
laboratory cultures of the marine bacterium Shewanella putrefaciens
strain MR-4.- Marine Chemistry 57, 3-4, 347-354.
Glaser, A. (2008): Ratgeber Meerwasserchemie
(Guide to marine chemistry).- Ruediger Latka Verlag, 206 pp (in
Haywood, P. & Vilbrandt, F. (1931): Iodine
content of shrimp waste.-J. Am. Chem. Soc. 53
Holmes-Farley, Randy (2003a): Iodine in Marine
Aquaria: Part I.- Advanced aquarist March 2003. http://advancedaquarist.com/issues/mar2003/chem.htm
Holmes-Farley, Randy (2003b): Iodine in Reef Tanks
2: Effects on Macroalgae Growth.- Advanced aquarist April 2003.
Liu et al. (1995): Requirements of shrimp, Penaeus
chinensis O'sbeck for potassium, sodium,magnesium and iodine.-
Chinese Journal of Oceanology and Limnology 13(2),
Pilson, M. E. Q. 1998. An Introduction to the
Chemistry of the Sea. Prentice-Hall, Inc. Upper Saddle River, NJ. 431
Sherill et al. (2004): Effects of ozonation on the
speciation of dissolved iodine in artificial seawater.- J. Zoo. Wildl.
Med. 35(3), 347-55.
Shimek, R.L. (2002a): It is still in the water.-
Reefkeeping 2002-03. http://reefkeeping.com/issues/2002-03/rs/feature/index.php
Shimek, R.L. (2002b): Down the drain. Exports from
Reef aquaria.- Reefkeeping 2002-12.http://reefkeeping.com/issues/2002-12/rs/feature/index.php
Solimabi et al. (1981) : Bromine and iodine
content in sponges and algae of the Andaman Sea.- Indian Journal
of Marine Sciences 10(3), 301-302.
Wong, George et. al. (2002): The transformation of iodate to iodide in marine phytoplankton cultures.- Marine Ecology: Progress Series 237, 27-39.