The marine reef aquarist carries out routine changes to maintain the necessary high quality of the seawater. The life within the aquarium makes demands on the seawater and the aquarist needs to ensure that there is sufficient quantity to meet those demands.
Some of the additions to the seawater are well known and accepted. One of these is calcium. Others are not so widely accepted as necessary, and one of these is iodine.
All non-scientific people, which includes me, tend to think of iodine as, well, iodine. As in so many things, it is not as simple as that.
In natural seawater the level of iodine is very low indeed, at 0.06 parts per million (ppm). As already stated, iodine is not just iodine. It appears in different types which come under the iodine heading: these can be iodide, iodate, hypoiodite, and molecular iodine. Looking at that lot, personally I prefer to stick to the iodine label!
The main iodine types in natural seawater (they are all present) are iodate and then iodide. Many algae types use iodate, and the zooxanthellae in the flesh of corals are algae, so it follows perhaps that iodate is of benefit to corals in that way.
It is also said that iodine (using the overall label) is a requirement of corals as it assists them resist UV radiation. It is also said that it helps in their protection against some parasites.
University research, and the studies of some commercial enterprises, are said to support the above benefits of iodine in seawater. It seems reasonable that as iodine is present in seawater, then some form of life may well have made use of it. I am certainly not qualified to agree or disagree with any research findings.
There is a lot of discussion and disagreement over the use of iodine. Some aquarists have shown pictures on the internet of their display aquariums, and they are indeed beautiful. However, in the technical descriptions a great many of them do not include iodine as an additive that they use.
I used to dose my soft coral reef with iodine, but stopped for a period of three months to see if there was any change (this is not scientific, any apparent result is anecdotal). As far as I was aware, there wasnâ€™t any change in the growth or health in the aquarium in general, never mind the corals. (I have to note that regular seawater changes at 10% are done and always have been, so there will have been some fresh iodine being introduced regularly, though I wouldnâ€˜t have thought the natural seawater level could have been maintained.) I didnâ€™t resume using an iodine additive. Now, two years later, I am going to do the reverse for three months, and see if I notice any change in anything. I expect I will not, but Iâ€™ll wait and see.
One thing that is clear. Iodine as stated is present in the seas and oceans in a very small amount. It is necessary to maintain the correct level in the aquarium, and overdosing could be disastrous. Therefore, bearing in mind that routine seawater changes are adding some iodine, it is necessary to test for the level. As the level is so low, a sensitive and reasonably accurate test kit is required. An ordinary hobbyist one will do nicely, but check it is suitable by obtaining opinion.
Whether the aquarist uses iodine as an additive or not seems to be optional at the moment. Perhaps in the course of time, scientific study will answer the questions about its use.
(Reference: Salifert, Nieuwgraaf, Holland)