Iodine is only one of the trace elements you will find in your water, but it plays a critical role in some of the biological processes of fish, invertebrates, and corals alike. It's one of the elements in the halogen group, along with chlorine and bromine. Due to the fact that iodine is found in water in 3 different forms (iodine, iodate, and iodide), testing for it can be tricky, but you can count on natural seawater having levels between .025 and .08ppm.
There is a lot of controversy surrounding whether or not dosing your tank for iodine is useful and/or necessary. Too high of levels can be dangerous to the tank. It has strong antibacterial properties and can kill off the bacteria that is so vital to keeping a healthy saltwater aquarium. However, it's also necessary to many species that are often found in these same aquariums. Various algae, including macroalgae species, use iodine. Furthermore, iodine is important for crustaceans, like shrimp, for the molting process. Many coral species use up iodine in the tank too - gorgonias, xenias, and sponges for instance.
For most, particularly those keeping fish only tanks or FOWLR tanks, regular water changes will keep your iodine levels exactly where they need to be. But if you have a tank that uses skimmers, chemical filtration, or has a high bio-load from macroalgae, coralline algae, crustaceans, or soft corals, it may be worth considering very careful, regular dosing, as these can all deplete iodine levels in your aquarium.
Though we don't make recommendations one way or the other for your tank, if you do decide that iodine supplements are best for your tank, there are a variety of options. We recommend the Red Sea Trace Colors supplements, as the instructions for use are very clear and easy to use, and Red Sea has done extensive research to make the best products possible. You can get the Iodine+ supplement on its own, or as part of a set with other vital trace element supplements.
Testing: If you're interested in finding out what your iodine levels (or other trace elements) are in your aquarium, try ICP analysis. Our analysis gives you levels for over 30 different elements in your water.