Iodine for reef tanks have 2 different functions. Most people use for dipping as a disinfectant to rid parasites/disease/etc.
Soft corals also benefit from iodine. Coralline needs a small amount as well if I recall. It also benefits some inverts such as shrimp as they need it for molting. Testing for iodine is a pain. No test kit is very reliable. I add 1-2ml every now and again if I see the xenia is shedding or the kenya tree doesn't open fully.
The first time I read the kent bottle, it said put 1 capful(5ml) per 50 gallons. I did that and then I read the second sentence. It said this is amount weekly and the preferred method is to drip it daily. The next minute my anemone shriveled up to a 2" ball (he's usually 12" across), my shrimp molted within minues after, and all my softies puffed up (each one doubling in size). Fish didn't care at all. I did a 25% water change and nem was fine next day. My softies remained puffed for a few days. The xenias grow like weeds already and iodine is a fertilizer for them.
If you do water changes often, the salt mix will have enough trace iodine that no addition is needed. Stuff like purple up, marine trace, etc. have iodine in them as well.
