Aquarium Iodine & Iodides
Inexperienced aquarists often see "bleaching" - corals and coraline algae losing color. Iodine and Iodides are the solution. They are absorbed to create the brilliant colors that inspired you to keep a reef aquarium in the first place! Depending on your bio-load, you may need more iodine and iodides than provided by your trace element supplements. Test to be sure you have enough.
Protein skimmers remove iodine within 24 hours - use a time-release iodine product, or turn off your skimmer for a while to let absorption to take place.