Iodide and Iodine Supplements
In seawater, Iodine measure at about 0.06 ppm. Although supplementing with iodine is controversial, it is very important because both the marine life and the protein skimmer use it up fairly quickly. It is generally cheaper to use the concentrated version which is dosed at 1 drop for each 25 gallons of aquarium water. Some experienced hobbyists use it on a regular basis while others won't use it at all.