Iodine is a very important trace element for the reef aquarium. It occurs in natural seawater at a concentration of .06 mg/l. Iodine is rapidly depleted in the closed system by protein skimming, activated carbon, and by use of all forms of marine algae, including zooxanthellae and calcareous algae. All forms of corals (stony, soft, mushroom anemones, anemones and polyps) use iodine. It can also be used as a germicidal and has been linked to u/v reducing pigments in corals. Mushroom anemones and Xenia especially respond well to regular iodine additions. Concentrations in the closed system can approach 10 mg/l without adverse effects, it is recommended not to exceed this level.